[Flang][OpenMP][Lower] Support lowering of `teams` directive to MLIR

This patch adds support for translating `teams` OpenMP directives to MLIR, when
appearing as either loop or block constructs and as part of combined constructs
or on its own.

The current Fortran parser does not allow the specification of the optional
lower bound for the "num_teams" clause, so only the `num_teams_upper` MLIR
argument is set by this patch.

Depends on D156809

Differential Revision: https://reviews.llvm.org/D156884