[clang] WIP: Fix MemberPointer serialization non-determinism

This fixes a problem originally reported here:
#132401 (comment)

This makes sure we only serialize the class declaration when its strictly
needed, and canonicalizes it, so it doesn't change in ways that don't round
trip.

There are no release notes, since this regression was never released.

Author: mizvekov

clang/include/clang/AST/TypeProperties.td

@@ -104,7 +104,8 @@ let Class = MemberPointerType in {
     let Read = [{ node->getQualifier() }];
   }
   def : Property<"Cls", DeclRef> {
-    let Read = [{ node->getMostRecentCXXRecordDecl() }];
+    let Read =
+        [{ node->isSugared() ? node->getMostRecentCXXRecordDecl()->getCanonicalDecl() : nullptr }];
   }
 
   def : Creator<[{