c++: Cleanup static init generation

The static init/fini generation is showing some bitrot.  This cleans
up several places to use C++, and also take advantage of already
having checked a variable for non-nullness.

	gcc/cp/
	* decl2.cc (ssdf_decl): Delete global.
	(start_static_storage_duration_function): Use some RAII.
	(do_static_initialization_or_destruction): Likewise.
	(c_parse_final_cleanups): Likewise.  Avoid rechecking 'vars'.