[Ada] Escalate pre-continuation messages from warnings to errors

[Pierre-Marie de Rodat via Gcc-patches]

When a compile-time constraint error is inserted into the AST, we emit
a main message (e.g. "value not in range of ...") and its continuation
(e.g. "Constraint_Error will be raised at run time"). When the main
message is emitted as a warning and the continuation is escalated into
an error, the main message should be escalated into an error as well.

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

gcc/ada/

        * errout.adb (Error_Msg_Internal): Propagate warnings-as-error
        flag from continuation message to its preceding messages.
diff --git a/gcc/ada/errout.adb b/gcc/ada/errout.adb
--- a/gcc/ada/errout.adb
+++ b/gcc/ada/errout.adb
@@ -1240,10 +1240,13 @@ package body Errout is
                       or else
                     Warning_Treated_As_Error (Get_Warning_Tag (Cur_Msg)));
 
-      --  Propagate Warn_Err to this message and preceding continuations
+      --  Propagate Warn_Err to this message and preceding continuations.
+      --  Likewise, propagate Is_Warning_Msg, because the current continued
+      --  message could have been escalated from warning to error.
 
       for J in reverse 1 .. Errors.Last loop
          Errors.Table (J).Warn_Err := Warn_Err;
+         Errors.Table (J).Warn     := Is_Warning_Msg;
          exit when not Errors.Table (J).Msg_Cont;
       end loop;