libstdc++: Set active member of union in std::string [PR103295]

libstdc++: Set active member of union in std::string [PR103295]

Clang diagnoses that the new constexpr std::string constructors are not
usable in constant expressions, because they start to write to members
of the union without setting an active member.

This adds a new helper function which returns the address of the local
buffer after making it the active member.

This doesn't fix all problems with Clang, because it still refuses to
write to memory returned by the allocator.

libstdc++-v3/ChangeLog:

	PR libstdc++/103295
	* include/bits/basic_string.h (_M_use_local_data()): New
	member function to make local buffer the active member.
	(assign(const basic_string&)): Use it.
	* include/bits/basic_string.tcc (_M_construct, reserve()):
	Likewise.