c++: tree walk into TYPENAME_TYPE.

While looking at 92583/92654 it occurred to me that typename types needed
the same fix.  So extract_locals_r also needs to see the TYPE_CONTEXT of a
TYPENAME_TYPE.  But it must not look through a typedef.

Most tree walking in the front end wants to walk through the syntactic form
of a type of expression, and doesn't care about the type referred to by a
typedef.  But min_vis_r does care.

gcc/cp/ChangeLog
2020-05-11  Jason Merrill  <jason@redhat.com>

	PR c++/92583
	PR c++/92654
	* tree.c (cp_walk_subtrees): Stop at typedefs.
	Handle TYPENAME_TYPE here.
	* pt.c (find_parameter_packs_r): Not here.
	(for_each_template_parm_r): Clear *walk_subtrees.
	* decl2.c (min_vis_r): Look through typedefs.