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

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

Clang still complains about using std::string in constexpr contexts due
to the changes made in commit 98a0d72a. This patch ensures that we set
the active member of the union as according to [class.union.general] p6.

libstdc++-v3/ChangeLog:

	PR libstdc++/103295
	* include/bits/basic_string.h (_M_use_local_data): Set active
	member to _M_local_buf.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
(cherry picked from commit 52672be7d328df50f9a05ce3ab44ebcae50fee1b)