[OpenMP] Avoid checking parent reference count in targetDataEnd
The patch has the following benefits:
- Eliminates a lock/unlock of the data mapping table.
- Clarifies the logic that determines whether a struct member's device-to-host transfer occurs. The old logic, which checks the parent struct's reference count, is a leftover from back when we had a different map interface (as pointed out at https://reviews.llvm.org/D104924#2846972).
Reviewed By: grokos
Differential Revision: https://reviews.llvm.org/D104924