ada: Better error message for bad general case statements

If -gnatX0 is specified, we allow case statements with a selector
expression of a record or array type, but not of a private type.
If the selector expression is of a private type then we should generate
an appropriate error message instead of a bugbox.

gcc/ada/

	* sem_ch5.adb (Analyze_Case_Statement): Emit a message and return
	early in the case where general case statements are allowed but
	the selector expression is of a private type. This is done to
	avoid a bugbox.