[FLANG][OpenMP] Support for Custom Reduction

Is there a plan or a timeline on when we will have custom openmp reduction support on FLANG. The following fails with several errors:

```
module mymod
type maxtype
real :: val
end type maxtype

!$omp declare reduction(mymax: maxtype : omp_out = getmax(omp_in,omp_out)) initializer(omp_priv=omp_orig)

contains
function getmax(m1, m2)
implicit none
!$omp declare target
type(maxtype), intent(in) :: m1, m2
type(maxtype) :: getmax

if (m1%val > m2%val) then
getmax%val = m1%val
else
getmax%val = m2%val
endif
end function getmax
end module mymod
```