[OpenMPIRBuilder] Add createOffloadMaptypes and createOffloadMapnames functions

Add function to create the offload_maptypes and the offload_mapnames globals. These two functions
are used in clang. They will be used in the Flang/MLIR lowering as well.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D101503