[Flang][OpenMP] Support for non-rectangular collapsed loops

This was introduced in OpenMP 5.0.[^1]

```fortran
program test_openmp_5
implicit none
integer, parameter :: N = 10
integer :: i, j
!$omp parallel do collapse(2) private(i, j)
do i = 1, N
do j = i, N
write(*,*) i, j
end do
end do
!$omp end parallel do
end program
```

Currently it errors with

```
error: Semantic errors in /app/example.f90
/app/example.f90:5:9: error: Trip count must be computable and invariant
!$omp parallel do collapse(2) private(i, j)
^^^^^^^^^^^
```

Live example: https://godbolt.org/z/jbro4es9h

[^1]: https://developers.redhat.com/blog/2021/05/03/new-features-in-openmp-5-0-and-5-1

Start with the example.f90 reproducer in the issue and run it through Flang, or use the linked Godbolt example, to confirm the current trip-count error. Compare the behavior with the OpenMP 5.0 non-rectangular collapse requirement; done means the sample is accepted and produces the expected collapsed-loop iterations.

Written by the indexing model from the issue text.