c++: deduction guide using alias [PR99180]

c++: deduction guide using alias [PR99180]

alias_ctad_tweaks was expecting that all deduction guides for the class
would be suitable for deduction from the alias definition; in this case, the
deduction guide uses 'true' and the alias B uses 'false', so deduction
fails.  But that's OK, we just don't use that deduction guide.  I also
noticed that we were giving up on deduction entirely if substitution failed
for some guide; we should only give up on that particular deduction guide.

We ought to give a better diagnostic about this case when deduction fails,
but that can wait.

gcc/cp/ChangeLog:

	PR c++/99180
	PR c++/93295
	PR c++/93867
	PR c++/99118
	PR c++/96873
	* pt.c (alias_ctad_tweaks): Handle failure better.

gcc/testsuite/ChangeLog:

	PR c++/99180
	PR c++/93295
	PR c++/93867
	PR c++/95486
	* g++.dg/cpp2a/class-deduction-alias5.C: New test.
	* g++.dg/cpp2a/class-deduction-alias6.C: New test.
	* g++.dg/cpp2a/class-deduction-alias7.C: New test.
	* g++.dg/cpp2a/class-deduction-alias8.C: New test.