"introduce no new bootstrap warning" criteria. was: Loop iv debugging patch

[dewar@gnat.com]

<<What happened to the idea of a pragma to mark particular warnings as
expected in code - whether globally, scoped with push/pop, or for a
particular line of code only?  (With the preferred design from previous
>>

Ada does not have this feature, but it allows an implementation to 
introduce new pragmas, and we introduced pragma Warnings (Off | On)
precisely for this purpose, and have found it very useful.

After all almost any warning has the property that it is wrong sometimes,
(otherwise it should be an error).