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
Joel E. Denny