[OpenMP] Fix `target data` exit for array extension

For example:

```
 #pragma omp target data map(tofrom:arr[0:100])
 {
   #pragma omp target exit data map(delete:arr[0:100])
   #pragma omp target enter data map(alloc:arr[98:2])
 }
```

Without this patch, the transfer at the end of the target data region
is broken and fails depending on the target device.  According to my
read of the spec, the transfer shouldn't even be attempted because
`arr[0:100]` isn't (fully) present there.  To fix that, this patch
makes `DeviceTy::getTgtPtrBegin` return null for this case.

Reviewed By: grokos

Differential Revision: https://reviews.llvm.org/D85342