[flang][OpenMP] Support for "declare variant"

Support for "declare variant" missing in flang:

e.g.

```fortran
subroutine foo_parallel_variant()
print *, "parallel variant"
end subroutine

subroutine foo_parallel()
!$omp declare variant(foo_parallel_variant) match(construct={parallel})
print *, "parallel"
end subroutine

!$omp parallel num_threads(2)
call foo_parallel()
!$omp end parallel
```

`error: loc("flang/declare_variant/declare_variant.f90":25:11): flang/lib/Lower/OpenMP/OpenMP.cpp:4106: not yet implemented: OmpDeclareVariantDirective`

I noticed this when working with "!$omp dispatch".