c++: uninstantiated template friends

c++: uninstantiated template friends

template friends need to be recognized by module streaming and
associated with the befriending class.  but their context is that of
the friend (a namespace or other class).  This adds a flag to mark
such templates, and uses their DECL_CHAIN to point at the befriender.

	gcc/cp
	* cp-tree.h (DECL_UNINSTANTIATED_TEMPLATE_FRIEND): New.
	* pt.c (push_template_decl): Set it.
	(tsubst_friend_function): Clear it.