[Ada] Fix failing assertions related to volatile objects

[Ada] Fix failing assertions related to volatile objects

gcc/ada/

	* sem_ch3.adb (Process_Discriminants): Set Ekind of the
	processed discriminant entity before passing to
	Is_Effectively_Volatile, which was crashing on a failed
	assertion.
	* sem_prag.adb (Analyze_External_Property_In_Decl_Part): Prevent
	call to No_Caching_Enabled with entities other than variables,
	which was crashing on a failed assertion.
	(Analyze_Pragma): Style cleanups.
	* sem_util.adb (Is_Effectively_Volatile): Enforce comment with
	an assertion; prevent call to No_Caching_Enabled with entities
	other than variables.
	(Is_Effectively_Volatile_Object): Only call
	Is_Effectively_Volatile on objects, not on types.
	(No_Caching_Enabled): Enforce comment with an assertion.