There are heuristics for suppressing warnings about unused objects in
trivial cases. In particular, we try to suppress warnings here:

   function F (A : Integer) return Some_Type;
      X : Some_Type;
   begin
      raise Not_Yet_Implemented;
      return X;
   end F;

But it doesn't work if Some_Type is controlled. This patch fixes that
bug.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch6.adb
	(Analyze_Subprogram_Body_Helper): Use First_Real_Statement to deal
	with this case. Note that First_Real_Statement is likely to be
	removed as part of this ticket, so this is a temporary fix.