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

[Maksim Ivanov via cfe-commits]

emaxx-google wrote:

> I think this is fine as a temporary hack, but please add a FIXME comment explaining that this is working around a deeper underlying issue.

Sure, done.

https://github.com/llvm/llvm-project/pull/137910