c++: non-dep decltype folding of concept-id C<Ts...> [PR123676]

c++: non-dep decltype folding of concept-id C<Ts...> [PR123676]

Here since the expression within the decltype C<Ts...> is not instantiation
dependent (we know its type is bool, and don't care about its value)
finish_decltype_type instantiates it immediately via the usual tsubst_expr
with NULL_TREE args.  During which however tsubst_pack_expansion isn't
prepared to handle such a substitution due to an overly strict assert.
This patch relaxes the assert accordingly.

	PR c++/123676

gcc/cp/ChangeLog:

	* pt.cc (tsubst_pack_expansion): Relax unsubsituted_packs
	assert to allow !processing_template_decl when args is
	NULL_TREE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-decltype5.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>