[OpenMP][OMPIRBuilder] Migrate code to emit target region functions from clang to OMPIRBuilder

This patch moves some of the logic on how to emit target region functions and
adds emitTargetRegionFunction to the OpenMPIRBuilder. Also the
OpenMPOffloadMandatory flag is added to the config.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D139634