libstdc++: Skip tag dispatching for _S_relocate in C++17

libstdc++: Skip tag dispatching for _S_relocate in C++17

In C++17 mode all callers of _S_relocate have already done:

  if constexpr (_S_use_relocate())

so we don't need to repeat that check and use tag dispatching to avoid
ill-formed instantiations.

libstdc++-v3/ChangeLog:

	* include/bits/stl_vector.h (vector::_S_do_relocate): Remove
	C++20 constexpr specifier.
	(vector::_S_relocate) [__cpp_if_constexpr]: Call __relocate_a
	directly without tag dispatching.