[OpenMP] Use IsHostPtr where needed for targetDataEnd

As discussed in D105990, without this patch, `targetDataEnd`
determines whether to transfer data or delete a device mapping (as
opposed to assuming it's in shared memory) using two different
conditions, each of which is broken for some cases:

1. `!(UNIFIED_SHARED_MEMORY && TgtPtrBegin == HstPtrBegin)`: The
   broken case is rare: the device and host might happen to use the
   same address for their mapped allocations.  I don't know how to
   write a test that's likely to reveal this case, but this patch does
   fix it, as discussed below.
2. `!UNIFIED_SHARED_MEMORY || HasCloseModifier`: There are at least
   two broken cases:
    1. The `close` modifier might have been specified on an `omp
      target enter data` but not the corresponding `omp target exit
      data`, which thus might falsely assume a mapping is in shared
      memory.  The test `unified_shared_memory/close_enter_exit.c`
      already has a missing deletion as a result, and this patch adds
      a check for that.  This patch also adds the new test
      `close_member.c` to reveal a missing transfer and deletion.
    2. Use of discrete memory might have been forced by
      `omp_target_associate_ptr`, as in the test
      `unified_shared_memory/api.c`.  In the current `targetDataEnd`
      implementation, this condition turns out not be used for this
      case: because the reference count is infinite, a transfer is
      possible only with an `always` modifier, and this condition is
      never used in that case.  To ensure it's never used for that
      case in the future, this patch adds the test
      `unified_shared_memory/associate_ptr.c`.

Fortunately, `DeviceTy::getTgtPtrBegin` already has a solution: it
reports whether the allocation was found in shared memory via the
variable `IsHostPtr`.

After this patch, `HasCloseModifier` is no longer used in
`targetDataEnd`, and I wonder if the `close` modifier is ever useful
on an `omp target data end`.

Reviewed By: grokos

Differential Revision: https://reviews.llvm.org/D107925