libstdc++: Add missing constraints to std::bit_cast [PR105027]

libstdc++: Add missing constraints to std::bit_cast [PR105027]

Our std::bit_cast was relying on the compiler to check for errors inside
__builtin_bit_cast, instead of checking them as constraints. That means
std::bit_cast was not SFINAE-friendly.

This fix uses a requires-clause, so for old versions of Clang without
concepts support the function will still be unconstrained. At some point
in future we can remove the #ifdef __cpp_concepts check and rely on all
compilers having full concepts support in C++20 mode.

libstdc++-v3/ChangeLog:

	PR libstdc++/105027
	* include/std/bit (bit_cast): Add constraints.
	* testsuite/26_numerics/bit/bit.cast/105027.cc: New test.