[MLIR, OpenMP] Translation of OpenMP barrier construct to LLVM IR

Summary:
This patch adds support for translation of the OpenMP barrier construct to LLVM
IR. The OpenMP IRBuilder is used for this translation. In this patch the code
for translation is added to the existing LLVM dialect translation to LLVM IR.

The patch includes code changes and a testcase.

Reviewers: jdoerfert, nicolasvasilache, ftynse, rriddle, mehdi_amini

Reviewed By: ftynse, rriddle, mehdi_amini

Differential Revision: https://reviews.llvm.org/D72962