[OpenMP][Flang][MLIR] Add lowering of TargetOp for host codegen to LLVM-IR

This patch adds lowering of TargetOps for the host. The lowering outlines the
target region function and uses the OpenMPIRBuilder support functions to emit
the function and call. Code generation for offloading will be done in later
patches.

Reviewed By: kiranchandramohan, jdoerfert, agozillon

Differential Revision: https://reviews.llvm.org/D147172