c++: ICE with bogus late return type [PR99803]

c++: ICE with bogus late return type [PR99803]

Here we ICE when compiling this code in C++20, because we're trying to
slam a 'typename' after the ->.  The cp_parser_template_id call just
before the spot I'm changing parsed A::template A<int> as a BASELINK
that contains a constructor, but make_typename_type crashes on that.

This patch makes make_typename_type more robust instead of checking
for is_overloaded_fn prior calling it.

gcc/cp/ChangeLog:

	PR c++/99803
	* decl.c (make_typename_type): Give an error and return when
	name is is_overloaded_fn.
	* parser.c (cp_parser_class_name): Don't check is_overloaded_fn
	before calling make_typename_type.

gcc/testsuite/ChangeLog:

	PR c++/99803
	* g++.dg/cpp2a/typename14.C: Don't expect particular error
	messages.
	* g++.dg/cpp2a/typename19.C: New test.