c++: Yet more name-lookup api simplification

c++: Yet more name-lookup api simplification

This patch deals with LOOKUP_HIDDEN, which originally meant 'find
hidden friends', but it's being pressed into service for not ignoring
lambda-relevant internals.  However these two functions are different.
(a) hidden friends can occur in block scope (very uncommon) and (b) it
had the semantics of stopping after the innermost enclosing
namepspace.  That's really suspect for the lambda case, but not
relevant there because we never get to namespace scope (I think).
Anyway, I've split the flag into two and adjusted the lambda callers
to just search block scope.  These two flags are added to the
LOOK_want enum class, which allows dropping another parameter from the
name lookup routines.

The remaining LOOKUP_$FOO flags in cp-tree.h are, I think, now all
related to features of overload resolution, conversion operators and
reference binding.  Nothing to do with /name/ lookup.

	gcc/cp/
	* cp-tree.h (LOOKUP_HIDDEN): Delete.
	(LOOKUP_PREFER_RVALUE): Adjust initializer.
	* name-lookup.h (enum class LOOK_want): Add HIDDEN_FRIEND and
	HIDDEN_LAMBDA flags.
	(lookup_name_real): Drop flags parm.
	(lookup_qualified_name): Drop find_hidden parm.
	* name-lookup.c (class name_lookup): Drop hidden field, adjust
	ctors.
	(name_lookup::add_overload): Check want for hiddenness.
	(name_lookup::process_binding): Likewise.
	(name_lookup::search_unqualified): Likewise.
	(identifier_type_value_1): Adjust lookup_name_real call.
	(set_decl_namespace): Adjust name_lookup ctor.
	(qualify_lookup): Drop flags parm, use want for hiddenness.
	(lookup_qualified_name): Drop find_hidden parm.
	(lookup_name_real_1): Drop flags parm, adjust qualify_lookup
	calls.
	(lookup_name_real): Drop flags parm.
	(lookup_name_nonclass, lookup_name): Adjust lookup_name_real
	calls.
	(lookup_type_scope_1): Adjust qualify_lookup calls.
	* call.c (build_operator_new_call): Adjust lookup_name_real call.
	(add_operator_candidates): Likewise.
	* coroutines.cc (morph_fn_to_coro): Adjust lookup_qualified_name
	call.
	* parser.c (cp_parser_lookup_name): Adjust lookup_name_real calls.
	* pt.c (check_explicit_specialization): Adjust
	lookup_qualified_name call.
	(deduction_guides_for): Likewise.
	(tsubst_friend_class): Adjust lookup_name_real call.
	(lookup_init_capture_pack): Likewise.
	(tsubst_expr): Likewise, don't look in namespaces.
	* semantics.c (capture_decltype): Adjust lookup_name_real.  Don't
	look in namespaces.
	libcc1/
	* libcp1plugin.cc (plugin_build_dependent_exp): Adjust
	lookup_name_real call.