[OpenMP][OpenMPIRBuilder] Add kernel launch codegen to emitTargetCall

This patch adds code emission in emitTargetCall to call the OpenMP runtime to
launch an kernel, and to call the fallback host implementation if the launch
fails.

Reviewed By: TIFitis, kiranchandramohan, jdoerfert

Differential Revision: https://reviews.llvm.org/D155633