libstdc++: Ensure active union member is correctly set

libstdc++: Ensure active union member is correctly set

This patch ensures that the union members for std::string and
std::variant are always properly set when a change occurs.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h: (basic_string(basic_string&&)):
	Activate _M_local_buf when needed.
	(basic_string(basic_string&&, const _Alloc&)): Likewise.
	* include/bits/basic_string.tcc: (basic_string::swap): Likewise.
	* include/std/variant: (__detail::__variant::__construct_n): New.
	(__detail::__variant::__emplace): Use __construct_n.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
(cherry picked from commit 28adad7a32ed92a3c0906b38411c4ed686bbacc5)