In Eval_Arithmetic_Op we set Raises_Constraint_Error flag on integer
division by zero, but we didn't set it for real division by zero.

This only affects GNATprove, because GNAT sets this flag inside
Apply_Compile_Time_Constraint_Error (which has an early return guarded
by GNATprove_Mode). Possibly the Raises_Constraint_Error flag should be
only set inside Apply_Compile_Time_Constraint_Error, but at least this
patch allows GNATprove to be have consistently for integer and real
division by zero.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_eval.adb (Eval_Arithmetic_Op): Call
	Set_Raises_Constraint_Error on real division by zero just like
	it is called for integer division by zero earlier in this
	routine.