libstdc++: Change std::variant union member to empty struct

libstdc++: Change std::variant union member to empty struct

This more clearly expresses the intent (a completely unused, trivial
type) than using char. It's also consistent with the unions in
std::optional.

libstdc++-v3/ChangeLog:

	* include/std/variant (_Uninitialized): Use an empty struct
	for the unused union member, instead of char.