ada: Allow component clauses for certain record components with discriminants

ada: Allow component clauses for certain record components with discriminants

Before this patch, the compiler rejected record representation clauses
when a component has a subtype with a non-static discriminant constraint.
This is a somewhat coarse-grained since the size of such a component can
be constant relatively to the discriminant. One such case in particular
is when the component subtype is an unchecked union.

This patch makes the compiler accept the unchecked union case.

gcc/ada/ChangeLog:

	* freeze.adb (Size_Known): Modify rejection condition.