c++: Static init guard generation

The guard generation for a static var init was overly verbose.  We can
use a bit of RAII and avoid some rechecking.  Also in the !cxa_atexit
case, the only difference is whether can become whether to use
post-inc or pre-dec.

	gcc/cp/
	* decl2.cc (fix_temporary_vars_context_r): Use data argument
	for new context.
	(one_static_initialization_or_destruction): Adjust tree walk
	call.  Refactor guard generation.