Remove discriminant checks in gigi

Remove discriminant checks in gigi

gcc/ada/
	* gcc-interface/utils.c (gnat_pushdecl): Fix typo in comment.
	* gcc-interface/utils2.c (build_simple_component_ref): Build NULL_EXPR
	if the offset of the field has overflowed.
	(build_component_ref): Add gigi checking assertion that the reference
	has been built and replace the discriminant check by a Program_Error.