[clang] Fix nondeterminism in MemberPointerType (llvm#137910)

This commit fixes the nondeterminism issue in C++ header module enabled builds which were observed after
llvm#132401.

The issue was related to the fact that the hash set operation in MemberPointerType::Profile() was triggering getMostRecentDecl(). As the latter may trigger the loading of new entities from the external AST source, this
was presumably causing reentrant modification of data structure or some other issue that affects
compiler's output in a nondeterministic way (likely depending on specific values hashes/pointers have).

The change should otherwise be a no-op, because whether we take a "most recent" or "any" Decl shouldn't
matter since `getCanonicalDecl()` is called on it anyway inside `MemberPointerType::Profile()`.

We haven't been able to come up with a deterministic regression test for this fix.

Author: emaxx-google

clang/include/clang/AST/Type.h

@@ -3602,6 +3602,9 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
   }
 
   NestedNameSpecifier *getQualifier() const { return Qualifier; }
+  /// Note: this can trigger extra deserialization when external AST sources are
+  /// used. Prefer `getCXXRecordDecl()` unless you really need the most recent
+  /// decl.
   CXXRecordDecl *getMostRecentCXXRecordDecl() const;
 
   bool isSugared() const;
@@ -3610,7 +3613,10 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
   }
 
   void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, getPointeeType(), getQualifier(), getMostRecentCXXRecordDecl());
+    // FIXME: `getMostRecentCXXRecordDecl()` should be possible to use here,
+    // however when external AST sources are used it causes nondeterminism
+    // issues (see https://github.com/llvm/llvm-project/pull/137910).
+    Profile(ID, getPointeeType(), getQualifier(), getCXXRecordDecl());
   }
 
   static void Profile(llvm::FoldingSetNodeID &ID, QualType Pointee,
@@ -3620,6 +3626,9 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
   static bool classof(const Type *T) {
     return T->getTypeClass() == MemberPointer;
   }
+
+private:
+  CXXRecordDecl *getCXXRecordDecl() const;
 };
 
 /// Capture whether this is a normal array (e.g. int X[4])

clang/lib/AST/Type.cpp

@@ -5305,10 +5305,14 @@ void MemberPointerType::Profile(llvm::FoldingSetNodeID &ID, QualType Pointee,
     ID.AddPointer(Cls->getCanonicalDecl());
 }
 
+CXXRecordDecl *MemberPointerType::getCXXRecordDecl() const {
+  return dyn_cast<MemberPointerType>(getCanonicalTypeInternal())
+      ->getQualifier()
+      ->getAsRecordDecl();
+}
+
 CXXRecordDecl *MemberPointerType::getMostRecentCXXRecordDecl() const {
-  auto *RD = dyn_cast<MemberPointerType>(getCanonicalTypeInternal())
-                 ->getQualifier()
-                 ->getAsRecordDecl();
+  auto *RD = getCXXRecordDecl();
   if (!RD)
     return nullptr;
   return RD->getMostRecentNonInjectedDecl();