Changes to std::variant to reduce code size

Changes to std::variant to reduce code size

	* include/std/variant (_Variant_storage<false, _Types...>::_M_reset):
	Replace raw visitation with a runtime check for the valueless state
	and a non-raw visitor.
	(_Variant_storage<false, _Types...>::_M_reset_impl): Remove.
	(variant::index()): Remove branch.
	(variant::swap(variant&)): Use valueless_by_exception() instead of
	comparing the index to variant_npos, and add likelihood attribute.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@271295 138bc75d-0d04-0410-961f-82ee72b054a4