PR libstdc++/91807
	* include/std/variant
	(_Copy_assign_base::operator=(const _Copy_assign_base&):
	Do the move-assignment from a temporary so that the temporary
	is constructed with an explicit index.
	* testsuite/20_util/variant/91807.cc: New.

(cherry picked from commit dbca7a69f276e4829354f87f2747ebff36f6090e)