[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