[clang] [Clang] Check constraints for an explicit instantiation of a member function (PR #104438)

[via cfe-commits]

cor3ntin wrote:

> > do you mean something like https://godbolt.org/z/4s6YEcrfa ?
> 
> I see both our godbolt links have a typo, but yes, I meant `A<2>::g`.

Oups. Anyway, I don't think we can do that as enable_if can look at the function argument.
(we don't do it in a bunch of other places either)

https://github.com/llvm/llvm-project/pull/104438