ada: Reject Valid_Value arguments originating from Standard

The constraint for Valid_Value not applying to types from Standard
should also apply to all types derived from those types.

gcc/ada/ChangeLog:

	* doc/gnat_rm/implementation_defined_attributes.rst: Update the
	documentation for Valid_Value.
	* sem_attr.adb (Analyze_Attribute): Reject types where
	the root type originates from Standard.
	* gnat_rm.texi: Regenerate.