find_parameter_packs_r doesn't look through typedefs, which is normally
correct, but that means we need to handle their declarations specially.

gcc/cp/ChangeLog
2020-03-14  Jason Merrill  <jason@redhat.com>

	PR c++/92909
	* pt.c (find_parameter_packs_r): [DECL_EXPR]: Walk
	DECL_ORIGINAL_TYPE of a typedef.