[OpenMP] [OMPIRBuilder] Create a new datatype to hold the unique target region info

Re-apply of: 3d0e9edd
Reverted in: 0cb65b0a

A function parameter was using the wrong type 'llvm::TargetRegion' instead of
'const llvm:: TargetRegion&', which caused the error in the address sanitizer.
The correct type is now used.

This patch puts the individual target region information attributes into a
struct so that the nested mappings are not needed and passing the information
around is simplified.

Reviewed By: jdoerfert, mikerice

Differential Revision: https://reviews.llvm.org/D136601