PR c++/93286 - ICE with __is_constructible and variadic template.

PR c++/93286 - ICE with __is_constructible and variadic template.

My GCC 10 patch for 93286 fixed the missing piece in tsubst's handling of
lists vs. that in tsubst_copy_and_build, but it would be better to share the
code between them.

gcc/cp/ChangeLog
2020-05-15  Jason Merrill  <jason@redhat.com>

	PR c++/93286 - ICE with __is_constructible and variadic template.
	* pt.c (tsubst_tree_list): New.
	(tsubst, tsubst_copy_and_build): Use it.
	* decl2.c (is_late_template_attribute): Handle error_mark_node args.