[MLIR] Add OpenMP dialect with barrier operation

Summary:
Barrier is a simple operation that takes no arguments and returns
nothing, but implies a side effect (synchronization of all threads)

Reviewers: jdoerfert

Subscribers: mgorny, guansong, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72400