[Ada] Additional error checking on index constraints with fixed-lower-bound ranges

[Ada] Additional error checking on index constraints with fixed-lower-bound ranges

gcc/ada/

	* sem_ch3.adb (Constrain_Array): Add error checking for
	fixed-lower-bound and constrained index ranges applied
	inappropriately on subtypes of unconstrained and
	fixed-lower-bound array types.
	(Constrain_Index): Correct and refine comment related to
	fixed-lower-bound index ranges.