c++: Avoid confusing 'nested' name

c++: Avoid confusing 'nested' name

instantiate_body has a local var call 'nested', which indicates that
this instantiation was caused during the body of some function -- not
necessarily its containing scope.  That's confusing, let's just use
'current_function_decl' directly.  Then we can also simplify the
push_to_top_level logic, which /does/ indicate whether this is an
actual nested function.  (C++ does not have nested functions, but OMP
ODRs fall into that category.  A follow up patch will use that more
usual meaning of 'nested' wrt to functions.)

	gcc/cp/
	* pt.c (instantiate_body): Remove 'nested' var, simplify
	push_to_top logic.