ada: Fix predicate failure that occurred in a test case

The CodePeer test case illustrating a problem where a "high"
precondition failure was expected, died in the GNAT FE on
input_reading.adb.  The problem was in Check_SCIL, where
it didn't properly handle a discriminant_specification.

gcc/ada/

	* sem_scil.adb: Handle discriminant specification.

• [DBH] gcc/ada/sem_scil.adb