[OpenMP] Make generating offloading entries more generic

This patch moves the logic for generating the offloading entries to the
OpenMPIRBuilder. This makes it easier to re-use in other places, such as
for OpenMP support in Flang or using the same method for generating
offloading entires for other languages like Cuda.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D123460