[flang][OpenMP] Remember decision of allowing past/future clause (#227419)
When a deprecated or a future clause is used on a directive, and it is
allowed with a warning, remember that decision and consider that clause
allowed on that directive in all subsequent checks.
Introduce OpenMPDeprecated and OpenMPFuture warning categories
to guard these warnings (and the corresponding `-Wopenmp-deprecated`
and `-Wopenmp-future` options).