c++: base-specifier name lookup is type-only [PR122192]

c++: base-specifier name lookup is type-only [PR122192]

The r13-6098 change to make TYPENAME_TYPE no longer always ignore
non-type bindings needs another exception: base-specifiers that are
represented as TYPENAME_TYPE, for which lookup must be type-only (by
[class.derived.general]/2).  This patch fixes this by giving such
TYPENAME_TYPEs a tag type of class_type rather than typename_type so
that we treat them like elaborated-type-specifiers (another type-only
lookup situation).

	PR c++/122192

gcc/cp/ChangeLog:

	* decl.cc (make_typename_type): Document base-specifier as
	another type-only lookup case.
	* parser.cc (cp_parser_class_name): Propagate tag_type to
	make_typename_type instead of hardcoding typename_type.
	(cp_parser_base_specifier): Pass class_type instead of
	typename_type as tag_type to cp_parser_class_name.

gcc/testsuite/ChangeLog:

	* g++.dg/template/dependent-base6.C: New test.

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