ada: Fix internal error on class-wide allocator inside if-expression

The problem is that the freeze node for the class-wide subtype built for the
expression of the allocator escapes from the dependent expression instead of
being stored in its list of actions.

gcc/ada/

	* freeze.adb (Freeze_Expression.Has_Decl_In_List): Deal specifically
	with itypes that are class-wide subtypes.