[Ada] Fix incorrect fixed-point computation in expression function

[Ada] Fix incorrect fixed-point computation in expression function

gcc/ada/

	* einfo.ads (E_Decimal_Fixed_Point_Subtype): Fix pasto.
	* freeze.adb (Freeze_Fixed_Point_Type): Retrieve the underlying type
	of the first subtype and do not use a stale value of Small_Value.
	* sem_res.adb (Resolve_Real_Literal): In the case of a fixed-point
	type, make sure that the base type is frozen and use its Small_Value
	to compute the corresponding integer value of the literal.