libstdc++: Implement P2655R3 changes to common_reference bullet 1

libstdc++: Implement P2655R3 changes to common_reference bullet 1

We implement this paper as a DR against C++20 (as do MSVC and libc++).

	PR libstdc++/120446

libstdc++-v3/ChangeLog:

	* include/bits/version.def (common_reference): New.
	* include/bits/version.h: Regenerate.
	* include/std/type_traits (__glibcxx_want_common_reference):
	Define.
	(__common_reference_impl<T1, T2, 1>): Add pointer convertibility
	constraints as per P2655R3.
	* testsuite/20_util/common_reference/p2655r3.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>