In some cases, the compiler builds a subtype entity Typ such that
Is_Scalar (Typ) is True and Is_Scalar (Base_Type (Typ)) is False.
This comes up in some cases involving a subtype of a private type,
where the full view of the private type is a scalar type. In such a
situation, it may also be the case that Is_Enumeration_Type (Typ) is True
and Is_Enumeration_Type (Base_Type (Typ)) is False. Some code incorrectly
assumed that if a subtype is known to be a scalar (respectively, enumeration)
type, then the same must be true of the base type of that subtype. Fix that
code to handle the case where that assumption does not hold.

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

gcc/ada/

	* exp_attr.adb
	(Attribute_Valid): Ensure that PBtyp is initialized to a value for
	which Is_Scalar_Type is True.
	* checks.adb
	(Determine_Range): Call Implemention_Base_Type instead of
	Base_Type in order to ensure that result is suitable for passing
	to Enum_Pos_To_Rep.