[Flang][OpenMP][OMPIRBuilder] Add lowering of TargetOp for device codegen

This patch adds support in the `OpenMPIRBuilder` for generating working
device code for OpenMP target regions. It generates and handles the
result of a call to `__kmpc_target_init()` at the beginning of the
function resulting from outlining each target region, and it also
generates the matching `__kmpc_target_deinit()` call before returning.

It relies on the implementation of target region outlining for host
codegen to handle the production of the new function and the lowering of
its body based on the contents of the associated target region.

Depends on D147172

Differential Revision: https://reviews.llvm.org/D147940