Fix spurious No_Elaboration violation for Size attribute

Fix spurious No_Elaboration violation for Size attribute

We optimize the associated range check but nevertheless flag a violation.

gcc/ada/
	* gcc-interface/trans.c (Call_to_gnu): Minor tweaks.
	(gnat_to_gnu_external): Likewise.
	(Raise_Error_to_gnu): Return an empty statement list if there is a
	condition and it is always false.
	(gnat_to_gnu): Do not check for elaboration code a priori during the
	translation but a posteriori instead.