[ADT] Extend EnableIfCallable for callables with incomplete returns

std::is_convertible has no defined behavior when its arguments
are incomplete, even if they are equal. In practice, it returns false.
Adding std::is_same allows us to use the constructor using a callable,
even if the return value is incomplete. We also check the case where
we convert a T into a const T.

Reviewed By: DaniilSuchkov

Differential Revision: https://reviews.llvm.org/D104703

Committer: Daniil Suchkov <dsuchkov@azul.com>