[clang][DebugInfo][gmodules] Set runtimeLang on ObjC forward declarations (#120154)

In Objective-C, forward declarations are currently represented as:
```
DW_TAG_structure_type
  DW_AT_name                ("Foo")
  DW_AT_declaration         (true)
  DW_AT_APPLE_runtime_class (DW_LANG_ObjC)
```
However, when compiling with `-gmodules`, when a class definition is
turned into a forward declaration within a `DW_TAG_module`, the DIE for
the forward declaration looks as follows:
```
DW_TAG_structure_type
  DW_AT_name                ("Foo")
  DW_AT_declaration         (true)
```

Note the absence of `DW_AT_APPLE_runtime_class`. With recent changes in
LLDB, not being able to differentiate between C++ and Objective-C
forward declarations has become problematic (see attached test-case and
explanation in https://github.com/llvm/llvm-project/pull/119860).

1 files changed, 6 insertions, 5 deletions

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 60f32f76..ff27690 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2995,20 +2995,21 @@ llvm::DIType *CGDebugInfo::CreateType(const ObjCInterfaceType *Ty,
   if (!ID)
     return nullptr;
 
+  auto RuntimeLang =
+      static_cast<llvm::dwarf::SourceLanguage>(TheCU->getSourceLanguage());
+
   // Return a forward declaration if this type was imported from a clang module,
   // and this is not the compile unit with the implementation of the type (which
   // may contain hidden ivars).
   if (DebugTypeExtRefs && ID->isFromASTFile() && ID->getDefinition() &&
       !ID->getImplementation())
-    return DBuilder.createForwardDecl(llvm::dwarf::DW_TAG_structure_type,
-                                      ID->getName(),
-                                      getDeclContextDescriptor(ID), Unit, 0);
+    return DBuilder.createForwardDecl(
+        llvm::dwarf::DW_TAG_structure_type, ID->getName(),
+        getDeclContextDescriptor(ID), Unit, 0, RuntimeLang);
 
   // Get overall information about the record type for the debug info.
   llvm::DIFile *DefUnit = getOrCreateFile(ID->getLocation());
   unsigned Line = getLineNumber(ID->getLocation());
-  auto RuntimeLang =
-      static_cast<llvm::dwarf::SourceLanguage>(TheCU->getSourceLanguage());
 
   // If this is just a forward declaration return a special forward-declaration
   // debug type since we won't be able to lay out the entire type.