c++: alias CTAD and aggregate deduction cand [PR98832]

c++: alias CTAD and aggregate deduction cand [PR98832]

During alias CTAD, we're accidentally ignoring the aggregate deduction
candidate for the underlying template because this guide is added
separately via maybe_aggr_guide (which doesn't yet handle alias
templates) instead of via deduction_guides_for (which does).  This patch
makes maybe_aggr_guide handle alias templates in a manner similar to
deduction_guides_for.

	PR c++/98832

gcc/cp/ChangeLog:

	* pt.c (maybe_aggr_guide): Handle alias templates appropriately.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias9.C: New test.

(cherry picked from commit c761be53f6b62e22ac5de18c4aaf88648f64f5b7)