[Ada] Set Error_Msg_Warn before use of << insertion

Calls to Error_Msg procedures with a message using the < or <<
insertions need to set appropriately Error_Msg_Warn. This was not done
in one call, which would lead to spurious errors when changing the
implementation of a runtime unit. This fixes it so that changes can be
applied.

gcc/ada/

	* exp_ch4.adb (Expand_Allocator_Expression): Set Error_Msg_Warn
	correctly.

gcc/ada/exp_ch4.adb

@@ -835,6 +835,7 @@ package body Exp_Ch4 is
               Make_Raise_Program_Error (Loc,
                 Reason => PE_Accessibility_Check_Failed));
 
+            Error_Msg_Warn := SPARK_Mode /= On;
             Error_Msg_N ("anonymous access discriminant is too deep for use"
                          & " in allocator<<", N);
             Error_Msg_N ("\Program_Error [<<", N);