[clang] Fix nondeterminism in MemberPointerType

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

The issue was related to the fact that the hash set operation in
MemberPointerType::Profile() was triggering getMostRecentDecl(). The
root cause seems to be that the latter was leading to the reentrant
modification of the hash set, with some probability (likely depending
on the actual values of hashes).

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 +3602,7 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
   }
 
   NestedNameSpecifier *getQualifier() const { return Qualifier; }
+  CXXRecordDecl *getCXXRecordDecl() const;
   CXXRecordDecl *getMostRecentCXXRecordDecl() const;
 
   bool isSugared() const;
@@ -3610,7 +3611,7 @@ class MemberPointerType : public Type, public llvm::FoldingSetNode {
   }
 
   void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, getPointeeType(), getQualifier(), getMostRecentCXXRecordDecl());
+    Profile(ID, getPointeeType(), getQualifier(), getCXXRecordDecl());
   }
 
   static void Profile(llvm::FoldingSetNodeID &ID, QualType Pointee,

clang/lib/AST/Type.cpp

@@ -5275,10 +5275,14 @@ 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();