[Flang][OpenMP] UNREACHABLE executed at llvm-project/flang/lib/Parser/parse-tree.cpp:297!

Reproducer:
```f90
program test
!$omp task depend(ii)
!$omp end task
end
```

Crash itself is reproducible on Godbolt:
https://godbolt.org/z/3jTYYxWT5

With assertion-enabled flang, the following output is obtained:
```txt
expecting modifiers on OmpDependClause::TaskDep
UNREACHABLE executed at /path_to_project/llvm-project/flang/lib/Parser/parse-tree.cpp:297!
```