[PR C++/59930] template friend classes & default args

[PR C++/59930] template friend classes & default args

https://gcc.gnu.org/ml/gcc-patches/2017-12/msg01055.html
	PR c++/59930
	* decl.c (xref_tag_1): Correct comments about template friends and
	default args.
	* friend.c (make_friend_class): Move comments concerning
	self-friendliness to code dealing with such.
	* pt.c (check_default_tmpl_args): Deal with template friend
	classes too.
	(push_template_decl_real): Check default args for non-function
	template friends.

	PR c++/59930
	* g++.dg/cpp0x/temp_default4.C: Adjust diagnostic.
	* g++.old-deja/g++.pt/friend23.C: Likewise.
	* g++.old-deja/g++.pt/friend24.C: Delete.

From-SVN: r255698