c++: Fix use of local in constexpr if.

c++: Fix use of local in constexpr if.

extract_local_specs wasn't finding the mention of 'an' as a template
argument because we weren't walking into template arguments.  So here I
changed cp_walk_subtrees to do so--only walking into template arguments in
the spelling of the type or expression, not any hidden behind typedefs.  The
change to use typedef_variant_p avoids looking through typedefs spelled with
'typedef' as well as those spelled with 'using'.  And then I removed some
now-redundant code for walking into template arguments in a couple of
walk_tree callbacks.

	PR c++/92654
	* tree.c (cp_walk_subtrees): Walk into type template arguments.
	* cp-tree.h (TYPE_TEMPLATE_INFO_MAYBE_ALIAS): Use typedef_variant_p
	instead of TYPE_ALIAS_P.
	* pt.c (push_template_decl_real): Likewise.
	(find_parameter_packs_r): Likewise.  Remove dead code.
	* error.c (find_typenames_r): Remove dead code.