[OpenMP] [Flang] [MLIR] Allow dist_schedule(static) to be default behavior in OMP DISTRIBUTE directive

[raghavendhra]

According to the OpenMP spec, the default schedule for distribute is implementation-defined. We effectively implement a static schedule by default, so we simply parse and ignore the dist_schedule clause instead of flagging it as unimplemented error message.

After this change,

!$omp target teams distribute parallel do dist_schedule(static) -> No longer issues unimplemented error message for dist_schedule static only case

!$omp target teams distribute parallel do dist_schedule(static, chunk_size) -> Issues unimplemented error message for dist_schedule chunk_size

[mjklemm]

LGTM

[skatrak]

LGTM, thanks!