[flang][openacc] fix bugs with default(none) checking (#149220)

A report of the following code not generating an error led to fixing two bugs in directive checking.

- We should treat CombinedConstructs as OpenACC Constructs
- We should treat DoConstruct index variables as private. 

```fortran
subroutine sub(nn)
  integer :: nn, ii
  !$acc serial loop default(none)
  do ii = 1, nn
  end do
  !$acc end serial loop
end subroutine
```
Here `nn` should be flagged as needing a data clause while `ii` should
still get one implicitly.

0 files changed, 0 insertions, 0 deletions