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

[Maksim Ivanov via cfe-commits]

emaxx-google wrote:

> Approving to land this and unblock our internal releases. Obviously, I encourage to address the issues raised by other reviewers in the follow-ups.
> 
> In particular, the most easily actionable one is about the naming of the getter from @mizvekov . Let's wait for the comments and fix that quickly if a better name is suggested.

Yes, I'll upload a PR for the rename immediately afterwards. Proceeding with the PR so that the particular nondeterminism issue we've been hitting is mitigated on tip-of-the-tree.

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