[c++] Add module attachment

[c++] Add module attachment

This adds module attachment as a distinct flag to 'in module purview'.
A declaration may have neither or both (as before), but can also have
just the 'in [named-module] purview', which was previously not
representable.

This new state allows some cleanup of redeclarations (particularly the
builtins), which was a little warty.  Some other internal APIs get
similarly clarified.

	gcc/cp/
	* cp-tree.h (DECL_MODULE_ATTACH_P): New.
	(struct lang_decl_base): Add module_attach_p flag.
	* decl.cc (duplicate_decls): Rework module redeclaration
	checking.
	* module.cc (trees_out::lang_decl_bools): Write attach flag.
	(trees_in::lang_decl_bools): ... and read it back.
	(trees_out::decl_value): Rework module attachment handling.
	(trees_in::decl_value): Rename local var to reflect meaning.
	(trees_in::key_mergeable): Likewise.
	(get_originating_module): Use DECL_MODULE_ATTACH_P.  No need
	to special-case mangling.
	(module_may_redeclare): Reimplement.
	(set_originating_module): Deal with attachment.
	* name-lookup.cc (maybe_record_mergeable_decl): Deal with
	attachment.
	(mergeable_namespace_slots): Likewise.
	(do_nonmember_using_decl): Likewise.
	* name-lookup.h (mergeable_namespace_slots): Adjust parm
	meaning.
	* ptree.cc (cxx_print_decl): Adjust purview & attach printing.