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

[via cfe-commits]

Sirraide wrote:

Actually, there is one more thing I just noticed. What about this (https://godbolt.org/z/vE4cEEEqs):
```c++
template <int N>
struct A {
    void g() __attribute__((enable_if(false, "foo"))) {}
};

template struct A<2>;
```
This currently also compiles, but should be ill-formed for the same reason. Not really related to concepts per se, but the fix should be in the same place.

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