ada: Tweak definition of Modulus field of entities

ada: Tweak definition of Modulus field of entities

The compiler may build modular integer subtypes whose base type is private
in the context of instantiations, but we want to be able to get the Modulus.

gcc/ada/ChangeLog:

	* einfo.ads (Modulus): Change to implementation base type only.
	* gen_il-gen-gen_entities.adb (Modular_Integer_Kind): Change type
	of Modulus field to Impl_Base_Type_Only.