[OpenMP] Fix segmentation fault when data field is used in is_device_pt

Currently, the field just emit map info for this pointer variable. It is
failed at run time. For the fields, the PartialStruct is created and it
needs call to emitCombinedEntry which create the base that covers all
the pieces.

The change is to generate map info as regular fields.

Differential Revision: https://reviews.llvm.org/D129608