[clang] [Clang][Sema] Ensure that the selected candidate for a member function explicit specialization is more constrained than all others (PR #101721)

[Richard Smith via cfe-commits]

https://github.com/zygoloid commented:

Changes to the algorithm for selecting the most constrained function look correct. It's not clear to me whether it's correct to stop calling `CheckMemberSpecialization` for member templates, though -- do we still do the necessary checks somewhere else?

https://github.com/llvm/llvm-project/pull/101721