c++: alias CTAD and copy deduction guide [PR115198]

c++: alias CTAD and copy deduction guide [PR115198]

Here we're neglecting to update DECL_NAME during the alias CTAD guide
transformation, which causes copy_guide_p to return false for the
transformed copy deduction guide since DECL_NAME is still __dguide_C
with TREE_TYPE C<B, T> but it should be __dguide_A with TREE_TYPE A<T>
(i.e. C<false, T>).  This ultimately results in ambiguity during
overload resolution between the copy deduction guide vs copy ctor guide.

This patch makes us update DECL_NAME of a transformed guide accordingly
during alias/inherited CTAD.

	PR c++/115198

gcc/cp/ChangeLog:

	* pt.cc (alias_ctad_tweaks): Update DECL_NAME of the transformed
	guides.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias22.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>