Reapply "[clang][DebugInfo] Emit DW_AT_object_pointer on function declarations with explicit `this`" (#123455)

This reverts commit c3a935e3f967f8f22f5db240d145459ee621c1e0.

The only change to the reverted commit is that this also updates
the OCaml bindings according to the C debug-info API changes.

The build failure originally introduced was:
```
FAILED: bindings/ocaml/debuginfo/debuginfo_ocaml.o /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.o
cd /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo && /usr/bin/ocamlfind ocamlc -c /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c -ccopt "-I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/bindings/ocaml/debuginfo/../llvm -D_GNU_SOURCE -D_DEBUG -D_GLIBCXX_ASSERTIONS -DEXPENSIVE_CHECKS -D_GLIBCXX_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/b/1/llvm-clang-x86_64-expensive-checks-debian/build/include -I/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/include  -DNDEBUG "
/b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c: In function ‘llvm_dibuild_create_object_pointer_type’:
/b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c:620:30: error: too few arguments to function ‘LLVMDIBuilderCreateObjectPointerType’
  620 |   LLVMMetadataRef Metadata = LLVMDIBuilderCreateObjectPointerType(
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bindings/ocaml/debuginfo/debuginfo_ocaml.c:23:
/b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/include/llvm-c/DebugInfo.h:880:17: note: declared here
  880 | LLVMMetadataRef LLVMDIBuilderCreateObjectPointerType(LLVMDIBuilderRef Builder,
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

1 files changed, 15 insertions, 6 deletions

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index f88f56c..6cbcaf0 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2016,13 +2016,15 @@ llvm::DISubroutineType *CGDebugInfo::getOrCreateInstanceMethodType(
   // First element is always return type. For 'void' functions it is NULL.
   Elts.push_back(Args[0]);
 
-  // "this" pointer is always first argument.
-  // ThisPtr may be null if the member function has an explicit 'this'
-  // parameter.
-  if (!ThisPtr.isNull()) {
+  const bool HasExplicitObjectParameter = ThisPtr.isNull();
+
+  // "this" pointer is always first argument. For explicit "this"
+  // parameters, it will already be in Args[1].
+  if (!HasExplicitObjectParameter) {
     llvm::DIType *ThisPtrType = getOrCreateType(ThisPtr, Unit);
     TypeCache[ThisPtr.getAsOpaquePtr()].reset(ThisPtrType);
-    ThisPtrType = DBuilder.createObjectPointerType(ThisPtrType);
+    ThisPtrType =
+        DBuilder.createObjectPointerType(ThisPtrType, /*Implicit=*/true);
     Elts.push_back(ThisPtrType);
   }
 
@@ -2030,6 +2032,13 @@ llvm::DISubroutineType *CGDebugInfo::getOrCreateInstanceMethodType(
   for (unsigned i = 1, e = Args.size(); i != e; ++i)
     Elts.push_back(Args[i]);
 
+  // Attach FlagObjectPointer to the explicit "this" parameter.
+  if (HasExplicitObjectParameter) {
+    assert(Elts.size() >= 2 && Args.size() >= 2 &&
+           "Expected at least return type and object parameter.");
+    Elts[1] = DBuilder.createObjectPointerType(Args[1], /*Implicit=*/false);
+  }
+
   llvm::DITypeRefArray EltTypeArray = DBuilder.getOrCreateTypeArray(Elts);
 
   return DBuilder.createSubroutineType(EltTypeArray, OriginalFunc->getFlags(),
@@ -5118,7 +5127,7 @@ llvm::DIType *CGDebugInfo::CreateSelfType(const QualType &QualTy,
   llvm::DIType *CachedTy = getTypeOrNull(QualTy);
   if (CachedTy)
     Ty = CachedTy;
-  return DBuilder.createObjectPointerType(Ty);
+  return DBuilder.createObjectPointerType(Ty, /*Implicit=*/true);
 }
 
 void CGDebugInfo::EmitDeclareOfBlockDeclRefVariable(