c++: Module-specific error and tree dumping

c++: Module-specific error and tree dumping

With modules, we need the ability to name 'foos' in different modules.
The idiom for that is a trailing '@modulename' suffix.  This adds that
to the error printing routines.  I also augment the tree dumping
machinery to show module-specific metadata.

	gcc/cp/
	* error.c (dump_module_suffix): New.
	(dump_aggr_type, dump_simple_decl, dump_function_name): Call it.
	* ptree.c (cxx_print_decl): Print module information.
	* module.cc (module_name, get_importing_module): Stubs.