ada: Update Controlling_Argument when copying trees

ada: Update Controlling_Argument when copying trees

When copying the AST we need to update fields that carry semantic
meaning and not just copy them. We already updated some of them,
e.g. the First/Next_Named_Association chain, but failed to update
the Controlling_Argument.

This fix doesn't appear to change anything for the compiler, but it is
needed for GNATprove, where we no longer want to expand expression
functions and instead we want to copy their preanalyzed expressions.

gcc/ada/

	* sem_util.ads (New_Copy_Tree): Update comment.
	* sem_util.adb (New_Copy_Tree): Update Controlling_Argument, very
	much like we update the First/Next_Named_Association.