c++: Mangling for modules

c++: Mangling for modules

This is the mangling changes for modules.  These were developed in
collaboration with clang, which also implemements the same ABI (or
plans to, I do not think the global init is in clang).  The global
init mangling is captured in
https://github.com/itanium-cxx-abi/cxx-abi/issues/99

	gcc/cp/
	* cp-tree.h (mangle_module_substitution, mangle_identifier)
	(mangle_module_global_init): Declare.
	* mangle.c (struct globals): Add mod field.
	 (mangle_module_substitution, mangle_identifier)
	(mangle_module_global_init): Define.
	(write_module, maybe_write_module): New.
	(write_name): Call it.
	(start_mangling): Clear mod field.
	(finish_mangling_internal): Adjust.
	* module.cc (mangle_module, mangle_module_fini)
	(get_originating_module): Stubs.