[ThinLTO][DebugInfo] Emit full type definitions when importing anonymous types. (#78461)

This fixes some cases of missing debuginfo caused by an interaction
between:


https://github.com/llvm/llvm-project/commit/f0d66559ea345db4b2116cae044aaf3399d7e829,
which drops the identifier from a DICompositeType in the module
containing its
vtable.

and


https://github.com/llvm/llvm-project/commit/a61f5e379675732666744bcba25efbc9922e016a,
which causes ThinLTO to import composite types as declarations when they
have
an identifier.

If a virtual class's DICompositeType has no identifier due to the first
change,
and contains a nested anonymous type which does have an identifier, then
the
second change can cause ThinLTO to output the classes's DICompositeType
as a
type definition that links to a non-defining declaration for the nested
type.
Since the nested anonyous type does not have a name, debuggers are
unable to
find the definition for the declaration.

Repro case:
```
cat > a.h <<EOF
class A {
 public:
  A();
  virtual ~A();

 private:
  union {
    int val;
  };
};
EOF

cat > a.cc <<EOF
#include "a.h"

A::A() { asm(""); }

A::~A() {}
EOF

cat > main.cc <<EOF
#include "a.h"

int main(int argc, char **argv) {
  A a;
  return 0;
}
EOF

clang++ -O2 -g -flto=thin -mllvm -force-import-all main.cc a.cc
gdb ./a.out -batch -ex 'pt /rmt A'
```

The gdb command outputs:
```
type = class A {
  private:
    union {
        <incomplete type>
    };
}
```

and dwarfdump -i a.out shows a DW_TAG_class_type for A with an
incomplete union
type (note that there is also a duplicate entry with the full union type
that
comes after).
```
< 1><0x0000001e>    DW_TAG_class_type
                      DW_AT_containing_type       <0x0000001e>
                      DW_AT_calling_convention    DW_CC_pass_by_reference
                      DW_AT_name                  (indexed string: 0x00000007)A
                      DW_AT_byte_size             0x00000010
                      DW_AT_decl_file             0x00000001 /path/to/./a.h
                      DW_AT_decl_line             0x00000001
...
< 2><0x0000002f>      DW_TAG_member
                        DW_AT_type                  <0x00000037>
                        DW_AT_decl_file             0x00000001 /path/to/./a.h
                        DW_AT_decl_line             0x00000007
                        DW_AT_data_member_location  8
< 2><0x00000037>      DW_TAG_union_type
                        DW_AT_export_symbols        yes(1)
                        DW_AT_calling_convention    DW_CC_pass_by_value
                        DW_AT_declaration           yes(1)
```

This change works around this by making ThinLTO always import full
definitions
for anonymous types.

1 files changed, 21 insertions, 16 deletions

diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index 910e9748..770eb83 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -1615,27 +1615,32 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     Metadata *Annotations = nullptr;
     auto *Identifier = getMDString(Record[15]);
     // If this module is being parsed so that it can be ThinLTO imported
-    // into another module, composite types only need to be imported
-    // as type declarations (unless full type definitions requested).
-    // Create type declarations up front to save memory. Also, buildODRType
-    // handles the case where this is type ODRed with a definition needed
-    // by the importing module, in which case the existing definition is
-    // used.
-    if (IsImporting && !ImportFullTypeDefinitions && Identifier &&
+    // into another module, composite types only need to be imported as
+    // type declarations (unless full type definitions are requested).
+    // Create type declarations up front to save memory. This is only
+    // done for types which have an Identifier, and are therefore
+    // subject to the ODR.
+    //
+    // buildODRType handles the case where this is type ODRed with a
+    // definition needed by the importing module, in which case the
+    // existing definition is used.
+    //
+    // We always import full definitions for anonymous composite types,
+    // as without a name, debuggers cannot easily resolve a declaration
+    // to its definition.
+    if (IsImporting && !ImportFullTypeDefinitions && Identifier && Name &&
         (Tag == dwarf::DW_TAG_enumeration_type ||
          Tag == dwarf::DW_TAG_class_type ||
          Tag == dwarf::DW_TAG_structure_type ||
          Tag == dwarf::DW_TAG_union_type)) {
       Flags = Flags | DINode::FlagFwdDecl;
-      if (Name) {
-        // This is a hack around preserving template parameters for simplified
-        // template names - it should probably be replaced with a
-        // DICompositeType flag specifying whether template parameters are
-        // required on declarations of this type.
-        StringRef NameStr = Name->getString();
-        if (!NameStr.contains('<') || NameStr.starts_with("_STN|"))
-          TemplateParams = getMDOrNull(Record[14]);
-      }
+      // This is a hack around preserving template parameters for simplified
+      // template names - it should probably be replaced with a
+      // DICompositeType flag specifying whether template parameters are
+      // required on declarations of this type.
+      StringRef NameStr = Name->getString();
+      if (!NameStr.contains('<') || NameStr.starts_with("_STN|"))
+        TemplateParams = getMDOrNull(Record[14]);
     } else {
       BaseType = getDITypeRefOrNull(Record[6]);
       OffsetInBits = Record[9];