[OpenMP] Improve ref count debug messages

[OpenMP] Improve ref count debug messages

For example, without this patch:

```
$ cat test.c
int main() {
  int x;
  #pragma omp target enter data map(alloc: x)
  #pragma omp target exit data map(release: x)
  ;
  return 0;
}
$ clang -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda test.c
$ LIBOMPTARGET_DEBUG=1 ./a.out |& grep 'Creating\|Mapping exists'
Libomptarget --> Creating new map entry with HstPtrBegin=0x00007ffcace8e448, TgtPtrBegin=0x00007f12ef600000, Size=4, Name=unknown
Libomptarget --> Mapping exists with HstPtrBegin=0x00007ffcace8e448, TgtPtrBegin=0x00007f12ef600000, Size=4, updated RefCount=1
```

There are two problems in this example:

* `RefCount` is not reported when a mapping is created, but it might
  be 1 or infinite.  In this case, because it's created by `omp target
  enter data`, it's 1.  Seeing that would make later `RefCount`
  messages easier to understand.
* `RefCount` is still 1 at the `omp target exit data`, but it's
  reported as `updated`.  The reason it's still 1 is that, upon
  deletions, the reference count is generally not updated in
  `DeviceTy::getTgtPtrBegin`, where the report is produced.  Instead,
  it's zeroed later in `DeviceTy::deallocTgtPtr`, where it's actually
  removed from the mapping table.

This patch makes the following changes:

* Report the reference count when creating a mapping.
* Where an existing mapping is reported, always report a reference
  count action:
    * `update suppressed` when `UpdateRefCount=false`
    * `incremented`
    * `decremented`
    * `deferred final decrement`, which replaces the misleading
      `updated` in the above example
* Add comments to `DeviceTy::getTgtPtrBegin` to explain why it does
  not zero the reference count.  (Please advise if these comments miss
  the point.)
* For unified shared memory, don't report confusing messages like
  `RefCount=` or `RefCount= updated` given that reference counts are
  irrelevant in this case.  Instead, just report `for unified shared
  memory`.
* Use `INFO` not `DP` consistently for `Mapping exists` messages.
* Fix device table dumps to print `INF` instead of `-1` for an
  infinite reference count.

Reviewed By: jhuber6, grokos

Differential Revision: https://reviews.llvm.org/D104559