[Ada] Allow confirming volatile properties on No_Caching variables

Volatile variables marked with the No_Caching aspect can now have
confirming aspects for other volatile properties, with a value of
False.

gcc/ada/

	* contracts.adb (Check_Type_Or_Object_External_Properties): Check
	the validity of combinations only when No_Caching is not used.
	* sem_prag.adb (Analyze_External_Property_In_Decl_Part): Check
	valid combinations with No_Caching.