[Clang][Flang][OpenMP] Add loadOffloadInfoMetadata and createOffloadEntriesAndInfoMet...

[Clang][Flang][OpenMP] Add loadOffloadInfoMetadata and createOffloadEntriesAndInfoMetadata into OMPIRBuilder's finalize and initialize

This allows the generation of OpenMP offload metadata for the OpenMP
dialect when lowering to LLVM-IR and moves some of the shared logic
between the OpenMP Dialect and Clang into the IRBuilder.

Reviewers: jsjodin, jdoerfert, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D148370