c++: alias ctad refinements [PR109321]

c++: alias ctad refinements [PR109321]

The two hunks fix missing handling demonstrated by the two testcases: first,
if we omit one alias template parm but include another, we need to rewrite
the deduced template args to reflect the new position of the included parm.
Second, if we can't deduce any template args for a parameter pack, it is
deduced to an empty pack.

	PR c++/109321
	PR c++/109320

gcc/cp/ChangeLog:

	* pt.cc (alias_ctad_tweaks): Rewrite deduced args.
	(type_targs_deducible_from): Handle null pack deduction.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias16.C: New test.
	* g++.dg/cpp2a/class-deduction-alias17.C: New test.