c++: DECL_FRIEND_P cleanup

c++: DECL_FRIEND_P cleanup

DECL_FRIEND_P's meaning has changed over time.  It now (almost) means
the the friend function decl has not been met via an explicit decl.
This completes that transition, renaming it to DECL_UNIQUE_FRIEND_P,
so one doesn't think it is the sole indicator of friendliness (plenty
of friends do not have the flag set).  This allows reduction in the
complexity of managing the field -- all in duplicate_decls now.

	gcc/cp/
	* cp-tree.h (struct lang_decl_fn): Adjust context comment.
	(DECL_FRIEND_P): Replace with ...
	(DECL_UNIQUE_FRIEND_P): ... this.  Only for FUNCTION_DECLs.
	(DECL_FRIEND_CONTEXT): Adjust.
	* class.c (add_implicitly_declared_members): Detect friendly
	spaceship from context.
	* constraint.cc (remove_constraints): Use a checking assert.
	(maybe_substitute_reqs_for): Use DECL_UNIQUE_FRIEND_P.
	* decl.c (check_no_redeclaration_friend_default_args):
	DECL_UNIQUE_FRIEND_P is signficant, not hiddenness.
	(duplicate_decls): Adjust DECL_UNIQUE_FRIEND_P clearing.
	(redeclaration_error_message): Use DECL_UNIQUE_FRIEND_P.
	(start_preparsed_function): Correct in-class friend processing.
	Refactor some initializers.
	(grokmethod): Directly check friend decl-spec.
	* decl2.c (grokfield): Check DECL_UNIQUE_FRIEND_P.
	* friend.c (do_friend): Set DECL_UNIQUE_FRIEND_P first, remove
	extraneous conditions.  Don't re set it afterwards.
	* name-lookup.c (lookup_elaborated_type_1): Simplify revealing
	code.
	(do_pushtag): Likewise.
	* pt.c (optimize_specialization_lookup_p): Check
	DECL_UNIQUE_FRIEND_P.
	(push_template_decl): Likewise.  Drop unneeded friend setting.
	(type_dependent_expression_p): Check DECL_UNIQUE_FRIEND_P.
	libcc1/
	* libcp1plugin.cc (plugin_add_friend): Set DECL_UNIQUE_FRIEND_P.