[OpenMP] Fix data sharing analysis in nested clause

Without this patch, clang doesn't complain that X needs explicit data
sharing attributes in the following:

```
 #pragma omp target teams default(none)
 {
   #pragma omp parallel num_threads(X)
     ;
 }
```

However, clang does produce that complaint after the braces are
removed.  With this patch, clang complains in both cases.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D55861

llvm-svn: 349635