[ADT] Add SFINAE guards to unique_function constructor.

We can't construct a working unique_function from an object that's not callable
with the right types, so don't allow deduction to succeed.
This avoids some ambiguous conversion cases, e.g. allowing to overload
on different unique_function types, and to conversion operators to
unique_function.

std::function and the any_invocable proposal have these.
This was added to llvm::function_ref in D88901 and followups

Differential Revision: https://reviews.llvm.org/D96794