c++: pack indexing is a non-deduced context [PR121795]

We weren't explicitly treating a pack index specifier as a non-deduced
context (as per [temp.deduct.type]/5), leading to an ICE for the first
testcase below.

	PR c++/121795

gcc/cp/ChangeLog:

	* pt.cc (unify) <case PACK_INDEX_TYPE>: New non-deduced context
	case.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/pack-indexing17.C: New test.
	* g++.dg/cpp26/pack-indexing17a.C: New test.

Reviewed-by: Marek Polacek <polacek@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 8e3230f4fed0d41e790d68c2534c057911bab35c)