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

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov approved this pull request.

LGTM

But please give it some time for @zygoloid to take a look in case we are missing anything else.

I see there are a few diagnostics sugar regressions, but they are not the fault of this patch, the issues are unrelated.

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