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.

gcc/ada/sem_scil.adb

@@ -91,6 +91,7 @@ package body Sem_SCIL is
             elsif Nkind (Ctrl_Tag) in N_Object_Renaming_Declaration
                                     | N_Object_Declaration
                                     | N_Parameter_Specification
+                                    | N_Discriminant_Specification
             then
                Ctrl_Typ := Etype (Defining_Identifier (Ctrl_Tag));