[Concepts] Check constraints for explicit template instantiations

[Concepts] Check constraints for explicit template instantiations

The standard requires[0] member function constraints to be checked when
explicitly instantiating classes. This patch adds this constraints
check.

This issue is tracked as #46029 [1].

Note that there's an related open CWG issue (2421[2]) about what to do when
multiple candidates have satisfied constraints. This is particularly an
issue because mangling doesn't contain function constraints, and so the
following code still ICEs with definition with same mangled name
'_ZN1BIiE1fEv' as another definition:

template<class T>
struct B {
  int f() requires std::same_as<T, int> {
    return 0;
  }
  int f() requires (std::same_as<T, int> &&
                    !std::same_as<T, char>) {
    return 1;
  }
};

template struct B<int>;

Also note that the constraints checking while instantiating *functions*
is still not implemented. I started looking at it but It's a bit more
complicated. I believe in such a case we have to consider the partial
constraints order and potentially choose the best candidate out of the
set of multiple valid ones.

[0]: https://eel.is/c++draft/temp.explicit#10
[1]: https://github.com/llvm/llvm-project/issues/46029
[2]: https://cplusplus.github.io/CWG/issues/2421.html

Differential Revision: https://reviews.llvm.org/D120255