libstdc++: Use deducing this in std::bind when available [PR80564]

libstdc++: Use deducing this in std::bind when available [PR80564]

Implement the forwarding performed by std::bind via deducing this when
available, instead of needing 4 operator() overloads.  Using deducing
this here is more complicated than in other standard call wrappers
because std::bind is not really "perfect forwarding": it doesn't
consider value category, and along with const-ness it also forwards
volatile-ness (until C++20).

The old implementation suffers from the same problem that other
pre-C++23 SFINAE-friendly call wrappers have which is solved by using
deducing this (see p5.5 of the deducing this paper P0847R7).

	PR libstdc++/80564

libstdc++-v3/ChangeLog:

	* include/std/functional (__cv_like): New.
	(_Bind::_Res_type): Don't define when not needed.
	(_Bind::__dependent): Likewise.
	(_Bind::_Res_type_cv): Likewise.
	(_Bind::operator()) [_GLIBCXX_EXPLICIT_THIS_PARAMETER]:
	Define as two instead of four overloads using deducing
	this.
	* testsuite/20_util/bind/cv_quals_2.cc: Ignore SFINAE
	diagnostics inside headers.
	* testsuite/20_util/bind/ref_neg.cc: Likewise.
	* testsuite/20_util/bind/80564.cc: New test.

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