Concretize gimple_call_set_fntype

Concretize gimple_call_set_fntype

This corresponds to:
  [PATCH 82/89] Concretize gimple_call_set_fntype
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01187.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> This is fine once prerequisites have gone in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00840.html

gcc/
	* gimple-fold.c (gimple_fold_builtin_sprintf_chk): Strengthen
	local "stmt" from gimple to gimple_call.

	* gimple.h (gimple_call_set_fntype): Require a gimple_call.

	* omp-low.c (lower_omp_1): Add a new local gimple_call "call_stmt",
	from a checked cast to gimple_call within the "case GIMPLE_CALL",
	for the regions where "stmt" is not subsequently overwritten.