Give a warning when no dwo files are provided (#94336)

In some scenarios based on the split-dwarf build process, the dwo file
is not generated as expected(That is to say, no dwo file path is stored
in the binary). When the llvm-dwp tool is called to generate the .dwp
file, it will exit without any warning.
 
So, the plan is to prompt a warning to tell the user that the dwo file
was not actually generated.
<img width="699" alt="image"
src="https://github.com/llvm/llvm-project/assets/150100070/5e5742f6-daad-450f-87e9-cb25449c3c7a">

2 files changed, 149 insertions, 1 deletions

diff --git a/llvm/test/tools/llvm-dwp/X86/empty_warning.s b/llvm/test/tools/llvm-dwp/X86/empty_warning.s
new file mode 100644
index 0000000..975b53f
--- /dev/null
+++ b/llvm/test/tools/llvm-dwp/X86/empty_warning.s
@@ -0,0 +1,145 @@
+# RUN: llvm-mc --triple=x86_64-unknown-linux --filetype=obj -dwarf-version=4 %s -o non_split.o
+# RUN: llvm-dwp -e non_split.o -o non_split.dwp 2>&1 | FileCheck %s --check-prefix=CHECK-WARNING
+
+# CHECK-WARNING: warning: executable file does not contain any references to dwo files
+
+.ifdef GEN
+;--- main.c
+int main() {
+    return 0;
+}
+;--- gen
+clang -g -S -gdwarf-4 main.c -o -
+.endif
+	.text
+	.file	"main.c"
+	.globl	main                            # -- Begin function main
+	.p2align	4, 0x90
+	.type	main,@function
+main:                                   # @main
+.Lfunc_begin0:
+	.file	1 "/proc/self/cwd" "main.c"
+	.loc	1 8 0                           # main.c:8:0
+	.cfi_startproc
+# %bb.0:                                # %entry
+	pushq	%rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset %rbp, -16
+	movq	%rsp, %rbp
+	.cfi_def_cfa_register %rbp
+	xorl	%eax, %eax
+	movl	$0, -4(%rbp)
+.Ltmp0:
+	.loc	1 9 5 prologue_end              # main.c:9:5
+	popq	%rbp
+	.cfi_def_cfa %rsp, 8
+	retq
+.Ltmp1:
+.Lfunc_end0:
+	.size	main, .Lfunc_end0-main
+	.cfi_endproc
+                                        # -- End function
+	.section	.debug_abbrev,"",@progbits
+	.byte	1                               # Abbreviation Code
+	.byte	17                              # DW_TAG_compile_unit
+	.byte	1                               # DW_CHILDREN_yes
+	.byte	37                              # DW_AT_producer
+	.byte	14                              # DW_FORM_strp
+	.byte	19                              # DW_AT_language
+	.byte	5                               # DW_FORM_data2
+	.byte	3                               # DW_AT_name
+	.byte	14                              # DW_FORM_strp
+	.byte	16                              # DW_AT_stmt_list
+	.byte	23                              # DW_FORM_sec_offset
+	.byte	27                              # DW_AT_comp_dir
+	.byte	14                              # DW_FORM_strp
+	.byte	17                              # DW_AT_low_pc
+	.byte	1                               # DW_FORM_addr
+	.byte	18                              # DW_AT_high_pc
+	.byte	6                               # DW_FORM_data4
+	.byte	0                               # EOM(1)
+	.byte	0                               # EOM(2)
+	.byte	2                               # Abbreviation Code
+	.byte	46                              # DW_TAG_subprogram
+	.byte	0                               # DW_CHILDREN_no
+	.byte	17                              # DW_AT_low_pc
+	.byte	1                               # DW_FORM_addr
+	.byte	18                              # DW_AT_high_pc
+	.byte	6                               # DW_FORM_data4
+	.byte	64                              # DW_AT_frame_base
+	.byte	24                              # DW_FORM_exprloc
+	.byte	3                               # DW_AT_name
+	.byte	14                              # DW_FORM_strp
+	.byte	58                              # DW_AT_decl_file
+	.byte	11                              # DW_FORM_data1
+	.byte	59                              # DW_AT_decl_line
+	.byte	11                              # DW_FORM_data1
+	.byte	73                              # DW_AT_type
+	.byte	19                              # DW_FORM_ref4
+	.byte	63                              # DW_AT_external
+	.byte	25                              # DW_FORM_flag_present
+	.byte	0                               # EOM(1)
+	.byte	0                               # EOM(2)
+	.byte	3                               # Abbreviation Code
+	.byte	36                              # DW_TAG_base_type
+	.byte	0                               # DW_CHILDREN_no
+	.byte	3                               # DW_AT_name
+	.byte	14                              # DW_FORM_strp
+	.byte	62                              # DW_AT_encoding
+	.byte	11                              # DW_FORM_data1
+	.byte	11                              # DW_AT_byte_size
+	.byte	11                              # DW_FORM_data1
+	.byte	0                               # EOM(1)
+	.byte	0                               # EOM(2)
+	.byte	0                               # EOM(3)
+	.section	.debug_info,"",@progbits
+.Lcu_begin0:
+	.long	.Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
+.Ldebug_info_start0:
+	.short	4                               # DWARF version number
+	.long	.debug_abbrev                   # Offset Into Abbrev. Section
+	.byte	8                               # Address Size (in bytes)
+	.byte	1                               # Abbrev [1] 0xb:0x40 DW_TAG_compile_unit
+	.long	.Linfo_string0                  # DW_AT_producer
+	.short	12                              # DW_AT_language
+	.long	.Linfo_string1                  # DW_AT_name
+	.long	.Lline_table_start0             # DW_AT_stmt_list
+	.long	.Linfo_string2                  # DW_AT_comp_dir
+	.quad	.Lfunc_begin0                   # DW_AT_low_pc
+	.long	.Lfunc_end0-.Lfunc_begin0       # DW_AT_high_pc
+	.byte	2                               # Abbrev [2] 0x2a:0x19 DW_TAG_subprogram
+	.quad	.Lfunc_begin0                   # DW_AT_low_pc
+	.long	.Lfunc_end0-.Lfunc_begin0       # DW_AT_high_pc
+	.byte	1                               # DW_AT_frame_base
+	.byte	86
+	.long	.Linfo_string3                  # DW_AT_name
+	.byte	1                               # DW_AT_decl_file
+	.byte	8                               # DW_AT_decl_line
+	.long	67                              # DW_AT_type
+                                        # DW_AT_external
+	.byte	3                               # Abbrev [3] 0x43:0x7 DW_TAG_base_type
+	.long	.Linfo_string4                  # DW_AT_name
+	.byte	5                               # DW_AT_encoding
+	.byte	4                               # DW_AT_byte_size
+	.byte	0                               # End Of Children Mark
+.Ldebug_info_end0:
+	.section	.debug_str,"MS",@progbits,1
+.Linfo_string0:
+	.byte	0                               # string offset=0
+.Linfo_string1:
+	.asciz	"main.c"                        # string offset=1
+.Linfo_string2:
+	.asciz	"/proc/self/cwd"                # string offset=8
+.Linfo_string3:
+	.asciz	"main"                          # string offset=23
+.Linfo_string4:
+	.asciz	"int"                           # string offset=28
+	.section	.GCC.command.line,"MS",@progbits,1
+	.zero	1
+	.ascii	"/usr/local/bin/clang-11 -Qn -g -S -gdwarf-4 main.c -o -"
+	.zero	1
+	.section	.debug_str,"MS",@progbits,1
+	.section	".note.GNU-stack","",@progbits
+	.addrsig
+	.section	.debug_line,"",@progbits
+.Lline_table_start0:
diff --git a/llvm/tools/llvm-dwp/llvm-dwp.cpp b/llvm/tools/llvm-dwp/llvm-dwp.cpp
index 4c4f2a9..18f4f1a 100644
--- a/llvm/tools/llvm-dwp/llvm-dwp.cpp
+++ b/llvm/tools/llvm-dwp/llvm-dwp.cpp
@@ -188,8 +188,11 @@ int llvm_dwp_main(int argc, char **argv, const llvm::ToolContext &) {
                         std::make_move_iterator(DWOs->end()));
   }
 
-  if (DWOFilenames.empty())
+  if (DWOFilenames.empty()) {
+    WithColor::defaultWarningHandler(make_error<DWPError>(
+        "executable file does not contain any references to dwo files"));
     return 0;
+  }
 
   std::string ErrorStr;
   StringRef Context = "dwarf streamer init";