[clang][DebugInfo][gmodules] Set runtimeLang on ObjC forward declarat…

…ions

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 llvm#119860).

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -2995,20 +2995,22 @@ 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);
+                                      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.

clang/test/Modules/ExtDebugInfo.m

@@ -75,7 +75,8 @@ int foo(ObjCClass *c) {
 
 // CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "ObjCClass",
 // CHECK-SAME:             scope: ![[MOD]],
-// CHECK-SAME:             flags: DIFlagFwdDecl)
+// CHECK-SAME:             flags: DIFlagFwdDecl,
+// CHECK-SAME:             runtimeLang: DW_LANG_ObjC)
 
 // CHECK-NOT: !DICompositeType(tag: DW_TAG_structure_type,
 // CHECK: !DICompositeType(tag: DW_TAG_enumeration_type,

clang/test/Modules/ModuleDebugInfo.m

@@ -39,6 +39,7 @@
 
 // CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "FwdDecl",
 // CHECK-SAME:             scope: ![[MODULE]],
+// CHECK-SAME:             runtimeLang: DW_LANG_ObjC
 
 // CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "ObjCClass",
 // CHECK-SAME:             scope: ![[MODULE]],