c++: pack indexing and if consteval

c++: pack indexing and if consteval

The pack index is manifestly constant-evaluated, and the call to
maybe_constant_value needs to reflect that or we wrongly complain about
non-constant index if the evaluation uses if consteval.

gcc/cp/ChangeLog:

	* semantics.cc (finish_type_pack_element): Pass mce_true to
	maybe_constant_value.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/pack-indexing16.C: New test.