libstdc++: Consolidate bullet 1 __common_reference_impl partial specs

libstdc++: Consolidate bullet 1 __common_reference_impl partial specs

... and in passing use requires-clauses instead of void_t based SFINAE.
This is a non-functional change that'll simplify implementing the
P2655R3 change to common_reference.

	PR c++/120446

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__common_reference_impl): Rewrite
	partial specializations to use requires-clause instead of
	an additional void_t template parameter.  Consolidate the
	partial specializations corresponding to bullet 1.

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