c++: Decl module-specific semantic processing

c++: Decl module-specific semantic processing

This adds the module-specific logic to the various declaration
processing routines in decl.c and semantic.c.  I also adjust the rtti
type creation, as those are all in the global module, so we need to
temporarily clear the module_kind, when they are being created.
Finally, I added init and fini module processing with the initialier
giving a fatal error if you try and turn it on (so don't do that yet).

	gcc/cp/
	* decl.c (duplicate_decls): Add module-specific redeclaration
	logic.
	(cxx_init_decl_processing): Export the global namespace, maybe
	initialize modules.
	(start_decl): Reject local-extern in a module, adjust linkage of
	template var.
	(xref_tag_1): Add module-specific redeclaration logic.
	(start_enum): Likewise.
	(finish_enum_value_list): Export unscoped members of an exported
	enum.
	(grokmethod): Implement p1779 linkage of in-class defined
	functions.
	* decl2.c (no_linkage_error): Imports are ok.
	(c_parse_final_cleanups): Call fini_modules.
	* lex.c (cxx_dup_lang_specific): Clear some module flags in the
	copy.
	* module.cc (module_kind): Define.
	(module_may_redeclare, set_defining_module): Stubs.
	(init_modules): Error on modules.
	(fini_modules): Stub.
	* rtti.c (push_abi_namespace): Save and reset module_kind.
	(pop_abi_namespace): Restore module kind.
	(build_dynamic_cast_1, tinfo_base_init): Adjust.
	* semantics.c (begin_class_definition): Add module-specific logic.
	(expand_or_defer_fn_1): Keep bodies of more fns when modules_p.