[OpenMP] Extend omp teams to permit nested omp atomic

OpenMP 5.2, sec. 10.2 "teams Construct", p. 232, L9-12 restricts what
regions can be strictly nested within a `teams` construct.  This patch
relaxes Clang's enforcement of this restriction in the case of nested
`atomic` constructs unless `-fno-openmp-extensions` is specified.
Cases like the following then seem to work fine with no additional
implementation changes:

```
 #pragma omp target teams map(tofrom:x)
 #pragma omp atomic update
 x++;
```

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D126323