[Ada] Ada2020 AI12-0282: Shared variable control aspects in generics

This patch further refines the checks and the corresponding error messsages on the legality of instantiations where some formal types and their corresponding actuals may carry Atomic, Volatile, etc. aspect specifications. The legality rules stated in RN 6.1 (12/5) are in fact backward-incompatibl

[Ada] Ada2020 AI12-0282: Shared variable control aspects in generics

This patch refines the checks and the corresponding error messsages on the legality of instantiations where some formal objects and their corresponding actuals may carry Atomic, Atonic_Components, Volatile, Volatile_Components, Independent, or Independent_Components aspect specifications. The lega