c++: DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P tweaks

c++: DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P tweaks

DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P templates can only appear as part
of a template friend declaration, and in turn get partially instantiated
only from tsubst_friend_function or tsubst_friend_class.  So rather than
having tsubst_template_decl clear the flag, let's leave it up to the
tsubst friend routines to clear it so that template friend handling stays
localized (note that tsubst_friend_function was already clearing it).

Also the template depth comparison test within tsubst_friend_function is
equivalent to DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P since such templates
belong to the class context (and so always have more levels than the
context), and conversely and it isn't possible to directly refer to an
existing template that has more levels than the class context.

gcc/cp/ChangeLog:

	* pt.cc (tsubst_friend_class): Simplify depth comparison test
	in the redeclaration code path to
	DECL_UNINSTANTIATED_TEMPLATE_FRIEND_P.  Clear the flag after
	partial instantiation here ...
	(tsubst_template_decl): ... instead of here.

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