[Ada] Don't allow REASON parameter for pragma Warnings in compiler unit

[Arnaud Charlet]

This patch forbids the use of the pragma Warnings REASON parameter in
compiler units (since this would cause bootstrap issues). This is for
internal development purposes only. No functionality change.

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

2013-09-10  Robert Dewar  <de...@adacore.com>

        * sem_prag.adb (Analyze_Pragma, case Warnings): Don't allow
        REASON parameter in compiler units (bootstrap issues).


Index: sem_prag.adb
===================================================================
--- sem_prag.adb        (revision 202462)
+++ sem_prag.adb        (working copy)
@@ -17958,6 +17958,10 @@
                then
                   Check_Arg_Is_Static_Expression (Last_Arg, Standard_String);
                   Arg_Count := Arg_Count - 1;
+
+                  --  Not allowed in compiler units (bootstrap issues)
+
+                  Check_Compiler_Unit (N);
                end if;
             end;