[llvm][DebugInfo] Attach object-pointer to DISubprogram declarations (#122742)

Currently Clang only attaches `DW_AT_object_pointer` to
`DW_TAG_subprogram` definitions. LLDB constructs C++ method types from
their `DW_TAG_subprogram` declaration, which is also the point at which
it needs to determine whether a method is static or not. LLDB's
heuristic for this could be very simple if we emitted
`DW_AT_object_pointer` on declarations. But since we don't, LLDB has to
guess whether an argument is an implicit object parameter based on the
DW_AT_name and DW_AT_type.

To simplify LLDB's job (and to eventually support C++23's explicit
object parameters), this patch adds the `DIFlagObjectPointer` to
`DISubprogram` declarations.

For reference, GCC attaches the object-pointer DIE to both the
definition and declaration: https://godbolt.org/z/3TWjTfWon

Fixes #120973

Author: Michael137

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

@@ -849,7 +849,9 @@ void DwarfUnit::constructTypeDIE(DIE &Buffer, const DIDerivedType *DTy) {
   }
 }
 
-void DwarfUnit::constructSubprogramArguments(DIE &Buffer, DITypeRefArray Args) {
+DIE *DwarfUnit::constructSubprogramArguments(DIE &Buffer, DITypeRefArray Args) {
+  // Args[0] is the return type.
+  DIE *ObjectPointer = nullptr;
   for (unsigned i = 1, N = Args.size(); i < N; ++i) {
     const DIType *Ty = Args[i];
     if (!Ty) {
@@ -860,8 +862,14 @@ void DwarfUnit::constructSubprogramArguments(DIE &Buffer, DITypeRefArray Args) {
       addType(Arg, Ty);
       if (Ty->isArtificial())
         addFlag(Arg, dwarf::DW_AT_artificial);
+      if (Ty->isObjectPointer()) {
+        assert(!ObjectPointer && "Can't have more than one object pointer");
+        ObjectPointer = &Arg;
+      }
     }
   }
+
+  return ObjectPointer;
 }
 
 void DwarfUnit::constructTypeDIE(DIE &Buffer, const DISubroutineType *CTy) {
@@ -1358,7 +1366,8 @@ void DwarfUnit::applySubprogramAttributes(const DISubprogram *SP, DIE &SPDie,
 
     // Add arguments. Do not add arguments for subprogram definition. They will
     // be handled while processing variables.
-    constructSubprogramArguments(SPDie, Args);
+    if (auto *ObjectPointer = constructSubprogramArguments(SPDie, Args))
+      addDIEEntry(SPDie, dwarf::DW_AT_object_pointer, *ObjectPointer);
   }
 
   addThrownTypes(SPDie, SP->getThrownTypes());

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.h

@@ -268,7 +268,9 @@ class DwarfUnit : public DIEUnit {
   void constructContainingTypeDIEs();
 
   /// Construct function argument DIEs.
-  void constructSubprogramArguments(DIE &Buffer, DITypeRefArray Args);
+  ///
+  /// \returns DIE of the object pointer if one exists. Nullptr otherwise.
+  DIE *constructSubprogramArguments(DIE &Buffer, DITypeRefArray Args);
 
   /// Create a DIE with the given Tag, add the DIE to its parent, and
   /// call insertDIE if MD is not null.