[Ada] Add debugging message

[Pierre-Marie de Rodat]

When raising Program_Error due to an incorrect Kind transition, print a
more informative debugging message.

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

2020-06-09  Bob Duff  <d...@adacore.com>

gcc/ada/

        * bindo-graphs.adb (Add_Edge_Kind_Check): Add the Image of the
        old and new Kinds to the raise Program_Error message.
--- gcc/ada/bindo-graphs.adb
+++ gcc/ada/bindo-graphs.adb
@@ -1824,7 +1824,7 @@ package body Bindo.Graphs is
          end case;
 
          if not OK then
-            raise Program_Error;
+            raise Program_Error with Kind'Img & "-->" & Attributes.Kind'Img;
          end if;
       end Add_Edge_Kind_Check;