[flang][OpenMP] upstream OpenMP lowering

Summary:
This patch implements lowering of OpenMP barrier construct from
pft to OpenMPDialect.

Patch is carved out of following merged PR's from fir-dev branch
of https://github.com/flang-compiler/f18-llvm-project/

PR's:
https://github.com/flang-compiler/f18-llvm-project/pull/248
https://github.com/flang-compiler/f18-llvm-project/pull/251

Unfortunately primary tool `bbc` for functional validation is not
yet upstreamed. So this patch includes a unittest for lowering
`!OMP barrier` construct.

Some part of the these PR's still remains downstream(functional test
and dialect registration to legalizer) for obvious reasons.
Will upstream them when the dependencies are upstreamed.

Reviewed By: schweitz, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D83659