[Flang][OpenMP] Add support for declare reduction without an initializer

```
program main
implicit none
integer :: i, n, result
integer, allocatable :: array(:)
!$omp declare reduction(add_points : integer: omp_out = omp_out + omp_in)

n = 100
allocate(array(n))
do i=1, n
array(i) = i
end do

result = 0
!$omp parallel do reduction(add_points : result )
do i = 1, 100
result = result + i
end do

print *, result
end program main
```

ClauseProcessor.cpp:611: not yet implemented: declare reduction without an initializer clause is not yet supported
LLVM ERROR: aborting