[Ada] Crash on task declaration with Restriction_Warning (No_Tasking)

[Pierre-Marie de Rodat]

Compiler aborts when the Restriction_Warning (No_Tasking) appears as a
configuration pragma, and the main unit includes a task declaration.

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

gcc/ada/

        * sem_prag.adb (Process_Restrictions_Or_Restriction_Warnings):
        when the restriction is a configuration pragma and specifies
        No_Tasking, a global flag is set to reject task declarations,
        and to prevent the construction of Master entities. The flag
        must not be set if the pragma is a Restriction_Warning, in which
        case task declarationns are allowed.
diff --git a/gcc/ada/sem_prag.adb b/gcc/ada/sem_prag.adb
--- a/gcc/ada/sem_prag.adb
+++ b/gcc/ada/sem_prag.adb
@@ -10438,10 +10438,13 @@ package body Sem_Prag is
                      Add_To_Config_Boolean_Restrictions (No_Elaboration_Code);
                   end if;
 
-               --  Special processing for No_Tasking restriction placed in
-               --  a configuration pragmas file.
+               --  Special processing for No_Tasking restriction (not just a
+               --  warning) when it appears as a configuration pragmas.
 
-               elsif R_Id = No_Tasking and then No (Cunit (Main_Unit)) then
+               elsif R_Id = No_Tasking
+                 and then No (Cunit (Main_Unit))
+                 and then not Warn
+               then
                   Set_Global_No_Tasking;
                end if;