c++: fix crash with pack indexing in noexcept [PR121325]

c++: fix crash with pack indexing in noexcept [PR121325]

In my r15-6792 patch I added a call to tsubst in tsubst_pack_index
to fully instantiate args#N in the pack.

Here we are in an unevaluated context, but since the pack is
a TREE_VEC, we call tsubst_template_args which has cp_evaluated
at the beginning.  That causes a crash because we trip on the
assert in tsubst_expr/PARM_DECL:

  gcc_assert (cp_unevaluated_operand);

because retrieve_local_specialization didn't find anything (becase
there are no local_specializations yet).

We can avoid the cp_evaluated by calling the new tsubst_tree_vec,
which creates a new TREE_VEC and substitutes each element.

	PR c++/121325

gcc/cp/ChangeLog:

	* pt.cc (tsubst_tree_vec): New.
	(tsubst_pack_index): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/pack-indexing18.C: New test.

Reviewed-by: Patrick Palka <ppalka@redhat.com>