[mlir][OpenMP] Generating enums in accordance with the guidelines

This patch changes the enums generated from `OMP.td` for MLIR according
to the enum naming guidelines in LLVM Coding Standards.

This also helps the issues we had with `static` being a C++ keyword and
also a value for the schedule clause.

Enumerator naming guidelines: https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D120825