ada: Simplify code for private types with unknown discriminants

ada: Simplify code for private types with unknown discriminants

Private type entities have Is_Constrained set when they have no
discriminants and no unknown discriminants; it is now set slightly
later, but simpler (this change could only affect Process_Discriminants,
but this flag should not be needed there).

Also, we now reuse this flag to detect private types with discriminants.

Code cleanup; behavior is unaffected.

gcc/ada/

	* sem_ch7.adb (New_Private_Type): Simplify setting of
	Is_Constrained flag.
	* sem_prag.adb (Is_Unconstrained_Or_Tagged_Item): Simplify
	detection of private types with no discriminant.