W/ Clang, compile/link C++ test programs with "-x c++"

Some testcases want to compile .c files with a C++ compiler.  So they
pass the "c++" option to gdb_compile.  That works fine with GCC, but
with Clang, it results in:

  gdb compile failed, clang-5.0: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]

and the testcase is skipped with UNTESTED.

A previous patch fixed a case like that in
gdb.compile/compile-cplus.exp, by adding -Wno-deprecated to the build
options.  However, there are other testcases that use the same
pattern, and all fail for the same reason.  For example:

 gdb.base/info-types-c++.exp
 gdb.base/max-depth-c++.exp
 gdb.base/msym-lang.exp
 gdb.base/whatis-ptype-typedefs.exp
 gdb.btrace/rn-dl-bind.exp

Fix this in a central place, within gdb_compile, by passing "-x c++"
to the compiler driver when we're compiling/linking C++.

This revealed that gdb.compile/compile-cplus.exp and
gdb.arch/amd64-entry-value-paramref.exp tests are compiling an
assembly file with the "c++" option, which would now fail to compile,
with the C++ compiler not grokking the assembly, of course.  We just
need to not pass "c++" and all the other related C++ options when
compiling an assembly file.

gdb/testsuite/ChangeLog:
2020-06-24  Pedro Alves  <palves@redhat.com>

	* gdb.arch/amd64-entry-value-paramref.exp: Use
	prepare_for_testing_full and don't pass "c++" for the .S file
	build spec.
	* gdb.compile/compile-cplus.exp: Don't compile $srcfile3 with
	$options, since it's an assembly file.  Remove -Wno-deprecated.
	* lib/gdb.exp (gdb_compile): Pass "-x c++" explicitly when
	compiling C++ programs.

4 files changed, 21 insertions, 12 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 6f4d99d..1b77459 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,15 @@
 2020-06-24  Pedro Alves  <palves@redhat.com>
 
+	* gdb.arch/amd64-entry-value-paramref.exp: Use
+	prepare_for_testing_full and don't pass "c++" for the .S file
+	build spec.
+	* gdb.compile/compile-cplus.exp: Don't compile $srcfile3 with
+	$options, since it's an assembly file.  Remove -Wno-deprecated.
+	* lib/gdb.exp (gdb_compile): Pass "-x c++" explicitly when
+	compiling C++ programs.
+
+2020-06-24  Pedro Alves  <palves@redhat.com>
+
 	* lib/gdb.exp (gdb_compile): Update intro comment.  If C/C++ with
 	Clang, add "-Wno-unknown-warning-option" to the options.
 
diff --git a/gdb/testsuite/gdb.arch/amd64-entry-value-paramref.exp b/gdb/testsuite/gdb.arch/amd64-entry-value-paramref.exp
index be60e25..cbb69f4 100644
--- a/gdb/testsuite/gdb.arch/amd64-entry-value-paramref.exp
+++ b/gdb/testsuite/gdb.arch/amd64-entry-value-paramref.exp
@@ -20,7 +20,8 @@ if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
     return
 }
 
-if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} "c++"] } {
+if { [prepare_for_testing_full "failed to prepare" \
+	  [list $testfile "c++" $srcfile {}]] } {
     return -1
 }
 
diff --git a/gdb/testsuite/gdb.compile/compile-cplus.exp b/gdb/testsuite/gdb.compile/compile-cplus.exp
index 85b2f20..f794e5a 100644
--- a/gdb/testsuite/gdb.compile/compile-cplus.exp
+++ b/gdb/testsuite/gdb.compile/compile-cplus.exp
@@ -24,11 +24,6 @@ if { [test_compiler_info gcc*] || [test_compiler_info clang*] } {
     lappend options additional_flags=-std=gnu++11
     lappend options c++
 }
-if [test_compiler_info clang*] {
-    # Treating C input as C++ is deprecated in Clang, so
-    # the build will fail without disabling -Wdeprecated.
-    lappend options additional_flags=-Wno-deprecated
-}
 
 if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
     verbose "Skipping x86_64 LOC_CONST test."
@@ -37,14 +32,9 @@ if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
 
 set srcfilesoptions [list ${srcfile} ${options}]
 if { $srcfile3 != "" } {
-    lappend srcfilesoptions $srcfile3 ${options}
+    lappend srcfilesoptions $srcfile3 {}
 }
 set srcfile4options "nodebug c++"
-if [test_compiler_info clang*] {
-    # Treating C input as C++ is deprecated in Clang, so
-    # the build will fail without disabling -Wdeprecated.
-    set srcfile4options "$srcfile4options additional_flags=-Wno-deprecated"
-}
 lappend srcfilesoptions $srcfile4 $srcfile4options
 if { [eval build_executable_from_specs ${testfile}.exp $testfile {$options} ${srcfilesoptions}] } {
     return -1
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 6b4f71b..8dbfa7e 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -3868,6 +3868,14 @@ proc gdb_compile {source dest type options} {
 	lappend new_options "additional_flags=-Wno-unknown-warning-option"
     }
 
+    # Treating .c input files as C++ is deprecated in Clang, so
+    # explicitly force C++ language.
+    if { [lsearch -exact $options getting_compiler_info] == -1
+	 && [lsearch -exact $options c++] != -1
+	 && [test_compiler_info "clang-*"]} {
+	lappend new_options additional_flags=-x\ c++
+    }
+
     # Place (and look for) Fortran `.mod` files in the output
     # directory for this specific test.
     if {[lsearch -exact $options f77] != -1 \