[OpenMP][OpenMPIRBuilder] Migrate kernel launch code and host fallback code...

[OpenMP][OpenMPIRBuilder] Migrate kernel launch code and host fallback code generation from Clang to the OpenMPIRBuilder

This patch refactors the code generation that emits the offloading kernel
launch and moves the core portion to the OpenMPIRBuilder so that it can be used
from flang in the future.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D151035