[Ada] Suppress warnings in trivial subprograms with finalization

["Marc Poulhiès" <poulhies@adacore.com>]

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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.

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diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -5409,14 +5409,20 @@ package body Sem_Ch6 is
       --  we have a special test to set X as apparently assigned to suppress
       --  the warning.
 
-      declare
-         Stm : Node_Id;
+      --  If X above is controlled, we need to use First_Real_Statement to skip
+      --  generated finalization-related code. Otherwise (First_Real_Statement
+      --  is Empty), we just get the first statement.
 
+      declare
+         Stm : Node_Id := First_Real_Statement (HSS);
       begin
+         if No (Stm) then
+            Stm := First (Statements (HSS));
+         end if;
+
          --  Skip call markers installed by the ABE mechanism, labels, and
          --  Push_xxx_Error_Label to find the first real statement.
 
-         Stm := First (Statements (HSS));
          while Nkind (Stm) in N_Call_Marker | N_Label | N_Push_xxx_Label loop
             Next (Stm);
          end loop;