[Ada] Add insertion character for overlay modification warnings

[Pierre-Marie de Rodat via Gcc-patches]

This enables tools that ingest GNAT's output to properly classify these
messages.

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

gcc/ada/

        * freeze.adb (Warn_Overlay): Add 'o' insertion character.
        * sem_ch13.adb (Analyze_Attribute_Definition_Clause): Likewise.
        * sem_util.adb (Note_Possible_Modifications): Likewise.
diff --git a/gcc/ada/freeze.adb b/gcc/ada/freeze.adb
--- a/gcc/ada/freeze.adb
+++ b/gcc/ada/freeze.adb
@@ -789,7 +789,7 @@ package body Freeze is
                then
                   Error_Msg_Sloc := Sloc (Addr);
                   Error_Msg_NE
-                    ("??constant& may be modified via address clause#",
+                    ("?o?constant& may be modified via address clause#",
                      Decl, O_Ent);
                end if;
             end;
@@ -10638,11 +10638,11 @@ package body Freeze is
          if Present (Old) then
             Error_Msg_Node_2 := Old;
             Error_Msg_N
-              ("default initialization of & may modify &??",
+              ("default initialization of & may modify &?o?",
                Nam);
          else
             Error_Msg_N
-              ("default initialization of & may modify overlaid storage??",
+              ("default initialization of & may modify overlaid storage?o?",
                Nam);
          end if;
 


diff --git a/gcc/ada/sem_ch13.adb b/gcc/ada/sem_ch13.adb
--- a/gcc/ada/sem_ch13.adb
+++ b/gcc/ada/sem_ch13.adb
@@ -6705,7 +6705,7 @@ package body Sem_Ch13 is
                     and then not Overlays_Constant (U_Ent)
                     and then Address_Clause_Overlay_Warnings
                   then
-                     Error_Msg_N ("??constant overlays a variable", Expr);
+                     Error_Msg_N ("?o?constant overlays a variable", Expr);
 
                   --  Imported variables can have an address clause, but then
                   --  the import is pretty meaningless except to suppress


diff --git a/gcc/ada/sem_util.adb b/gcc/ada/sem_util.adb
--- a/gcc/ada/sem_util.adb
+++ b/gcc/ada/sem_util.adb
@@ -25893,7 +25893,7 @@ package body Sem_Util is
 
                   Error_Msg_Sloc := Sloc (Addr);
                   Error_Msg_NE
-                    ("??constant& may be modified via address clause#",
+                    ("?o?constant& may be modified via address clause#",
                      N, O_Ent);
                end;
             end if;