[Flang] Add OpenMP Conversion patterns

This patch adds the OpenMP conversion patterns to the FIR to LLVM
dialect lowering pass in Codegen. Appropriate legalization
conditions are also added. This ensures that a mix of FIR and OpenMP
dialects can be lowered to LLVM and OpenMP  dialects. Also adds two
tests.

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project

Reviewed By: clementval, peixin

Differential Revision: https://reviews.llvm.org/D121793



Co-authored-by: Sourabh Singh Tomar <SourabhSingh.Tomar@amd.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>