[OpenMP] Fix `present` for exit from `omp target data`

Without this patch, the following example fails but shouldn't
according to OpenMP TR8:

```
 #pragma omp target enter data map(alloc:i)
 #pragma omp target data map(present, alloc: i)
 {
   #pragma omp target exit data map(delete:i)
 } // fails presence check here
```

OpenMP TR8 sec. 2.22.7.1 "map Clause", p. 321, L23-26 states:

> If the map clause appears on a target, target data, target enter
> data or target exit data construct with a present map-type-modifier
> then on entry to the region if the corresponding list item does not
> appear in the device data environment an error occurs and the
> program terminates.

There is no corresponding statement about the exit from a region.
Thus, the `present` modifier should:

1. Check for presence upon entry into any region, including a `target
   exit data` region.  This behavior is already implemented correctly.

2. Should not check for presence upon exit from any region, including
   a `target` or `target data` region.  Without this patch, this
   behavior is not implemented correctly, breaking the above example.

In the case of `target data`, this patch fixes the latter behavior by
removing the `present` modifier from the map types Clang generates for
the runtime call at the end of the region.

In the case of `target`, we have not found a valid OpenMP program for
which such a fix would matter.  It appears that, if a program can
guarantee that data is present at the beginning of a `target` region
so that there's no error there, that data is also guaranteed to be
present at the end.  This patch adds a comment to the runtime to
document this case.

Reviewed By: grokos, RaviNarayanaswamy, ABataev

Differential Revision: https://reviews.llvm.org/D84422