[OpenMP] Ensure broken assumptions print once, not thousands of times.

If we have a broken assumption we want to print a message to the user.
If the assumption is broken by many threads in many teams this can
become a problem. To avoid it we use a hash that tracks if a broken
assumption has (likely) been printed and avoid printing it again. This
is not fool proof and has some caveats that might cause problems in
the future (see comment) but it should improve the situation
considerably for now.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D112156