Attribute Valid_Scalars applied to prefix with an unchecked union
component causes a legitimate crash in the backend. It was already
rejected in the frontend for prefixes of a public type; now it is
rejected in the frontend regardless of the type privacy.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_attr.adb (Analyze_Attribute [Valid_Scalars]): Move check for
	unchecked union before checks for private and public types.