[MLIR, OpenMP] Support for flush operation, and translating the same to LLVM IR

Summary:
This patch adds support for flush operation in OpenMP dialect and translation of this construct to LLVM IR.
The OpenMP IRBuilder is used for this translation.
The patch includes code changes and testcase modifications.

Reviewed By: ftynse, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D79937