[OpenMP] Fix transformed loop's var privacy

Without this patch, the following example crashes Clang:

```
 #pragma omp target map(i)
 #pragma omp tile sizes(2)
 for (i = 0; i < N; ++i)
   ;
```

This patch fixes the crash by changing `Sema::isOpenMPPrivateDecl` not
to identify `i` as private just because it's the loop variable of a
`tile` construct.

While OpenMP TR11 and earlier do specify privacy for loop variables of
loops *generated* from a `tile` construct, I haven't found text
stating that the original loop variable must be private in the above
example, so this patch leaves it shared.  Even so, it is a bit
unexpected that value of `i` after the loop is `N - 1` instead of `N`.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D151356