[OpenMP] Avoid checking parent reference count in targetDataBegin

This patch is an attempt to do for `targetDataBegin` what D104924 does
for `targetDataEnd`:

* Eliminates a lock/unlock of the data mapping table.
* Clarifies the logic that determines whether a struct member's
  host-to-device 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>).

Additionally, it eliminates the `DeviceTy::getMapEntryRefCnt`, which
is no longer used after this patch.

While D104924 does not change the computation of `IsLast`, I found I
needed to change the computation of `IsNew` for this patch.  As far as
I can tell, the change is correct, and this patch does not cause any
additional `openmp` tests to fail.  However, I'm not sure I've thought
of all use cases.  Please advise.

Reviewed By: jdoerfert, jhuber6, protze.joachim, tianshilei1992, grokos, RaviNarayanaswamy

Differential Revision: https://reviews.llvm.org/D105121