libstdc++: Define C++26 member visit for std::variant [PR110356]

Implement the std::variant changes from P2637R3.

libstdc++-v3/ChangeLog:

	PR libstdc++/110356
	* include/bits/version.def (variant): Update for C++26.
	* include/bits/version.h: Regenerate.
	* include/std/variant (variant::visit): New member functions.
	* testsuite/20_util/variant/visit.cc: Check second alternative.
	* testsuite/20_util/variant/visit_member.cc: New test.