c++: P0847R7 (deducing this) - CWG2586 [PR102609]

This adds support for defaulted comparison operators and copy/move
assignment operators, as well as allowing user defined xobj copy/move
assignment operators. It turns out defaulted comparison operators already
worked though, so this just adds a test for them. Defaulted comparison
operators were not so nice and required a bit of a hack. Should work fine
though!

The diagnostics leave something to be desired, and there are some things
that could be improved with more extensive design changes. There are a few
notes left indicating where I think we could make improvements.

Aside from some small bugs, with this commit xobj member functions should be
feature complete.

	PR c++/102609

gcc/cp/ChangeLog:

	PR c++/102609
	C++23 P0847R7 (deducing this) - CWG2586.
	* decl.cc (copy_fn_p): Accept xobj copy assignment functions.
	(move_signature_fn_p): Accept xobj move assignment functions.
	* method.cc (do_build_copy_assign): Handle defaulted xobj member
	functions.
	(defaulted_late_check): Comment.
	(defaultable_fn_check): Comment.

gcc/testsuite/ChangeLog:

	PR c++/102609
	C++23 P0847R7 (deducing this) - CWG2586.
	* g++.dg/cpp23/explicit-obj-basic6.C: New test.
	* g++.dg/cpp23/explicit-obj-default1.C: New test.
	* g++.dg/cpp23/explicit-obj-default2.C: New test.

Signed-off-by: Waffl3x <waffl3x@protonmail.com>