c++: Cleanup some decl pushing apis

In cleaning up local decl handling, here's an initial patch that takes
advantage of C++'s default args for the is_friend parm of pushdecl,
duplicate_decls and push_template_decl_real and the scope & tpl_header
parms of xref_tag.  Then many of the calls simply not mention these.
I also rename push_template_decl_real to push_template_decl, deleting
the original forwarding function.  This'll make my later patches
changing their types less intrusive.  There are 2 functional changes:

1) push_template_decl requires is_friend to be correct, it doesn't go
checking for a friend function (an assert is added).

2) debug_overload prints out Hidden and Using markers for the overload set.

	gcc/cp/
	* cp-tree.h (duplicate_decls): Default is_friend to false.
	(xref_tag): Default tag_scope & tpl_header_p to ts_current & false.
	(push_template_decl_real): Default is_friend to false.  Rename to
	...
	(push_template_decl): ... here.  Delete original decl.
	* name-lookup.h (pushdecl_namespace_level): Default is_friend to
	false.
	(pushtag): Default tag_scope to ts_current.
	* coroutines.cc (morph_fn_to_coro): Drop default args to xref_tag.
	* decl.c (start_decl): Drop default args to duplicate_decls.
	(start_enum): Drop default arg to pushtag & xref_tag.
	(start_preparsed_function): Pass DECL_FRIEND_P to
	push_template_decl.
	(grokmethod): Likewise.
	* friend.c (do_friend): Rename push_template_decl_real calls.
	* lambda.c (begin_lamnbda_type): Drop default args to xref_tag.
	(vla_capture_type): Likewise.
	* name-lookup.c (maybe_process_template_type_declaration): Rename
	push_template_decl_real call.
	(pushdecl_top_level_and_finish): Drop default arg to
	pushdecl_namespace_level.
	* pt.c (push_template_decl_real): Assert no surprising friend
	functions.  Rename to ...
	(push_template_decl): ... here.  Delete original function.
	(lookup_template_class_1): Drop default args from pushtag.
	(instantiate_class_template_1): Likewise.
	* ptree.c (debug_overload): Print hidden and using markers.
	* rtti.c (init_rtti_processing): Drop refault args from xref_tag.
	(build_dynamic_cast_1, tinfo_base_init): Likewise.
	* semantics.c (begin_class_definition): Drop default args to
	pushtag.
	gcc/objcp/
	* objcp-decl.c (objcp_start_struct): Drop default args to
	xref_tag.
	(objcp_xref_tag): Likewise.
	libcc1/
	* libcp1plugin.cc (supplement_binding): Drop default args to
	duplicate_decls.
	(safe_pushtag): Drop scope parm.  Drop default args to pushtag.
	(safe_pushdecl_maybe_friend): Rename to ...
	(safe_pushdecl): ... here. Drop is_friend parm.  Drop default args
	to pushdecl.
	(plugin_build_decl): Adjust safe_pushdecl & safe_pushtag calls.
	(plugin_build_constant): Adjust safe_pushdecl call.