[Ada] Fix confusing error expression on an unknown restriction

[Ada] Fix confusing error expression on an unknown restriction

When pragma Restriction is used with an unknown restriction identifier,
it is better to not process the restriction expression, as it will
likely produce confusing error message.

In particular, an odd message appeared when there was a typo in the
restriction identifier whose expression requires special processing
(e.g.  No_Dependence_On instead of No_Dependence).

gcc/ada/

	* sem_prag.adb (Process_Restrictions_Or_Restriction_Warnings):
	Do not process expression of unknown restrictions.