ada: Support fixed-lower-bound array types as generic actual parameters

ada: Support fixed-lower-bound array types as generic actual parameters

Attempting to use a fixed-lower-bound array type (or subtype) as
an actual parameter for a formal unconstrained array type was being
rejected by the compiler (complaining about the index type of the
actual not matching the index type of the formal type).  The compiler
was improperly testing the actual's FLB range and finding that it didn't
statically match the index type of the formal array type; it should
instead test the underlying index type of the FLB type or subtype.

gcc/ada/ChangeLog:

	* sem_ch3.adb (Constrain_Index): In the case of a fixed-lower-bound index,
	set Etype of the newly created itype's Scalar_Range from the index's Etype.
	* sem_ch12.adb (Validate_Array_Type_Instance): If the actual subtype is
	a fixed-lower-bound type, then check again the Etype of its Scalar_Range.