[flang] Support `do concurrent ... reduce` for associating names (#148597)
Extends reduction support for `do concurrent`, in particular, for
associating names. Consider the following input:
```fortran
subroutine dc_associate_reduce
integer :: i
real, allocatable, dimension(:) :: x
associate(x_associate => x)
do concurrent (i = 1:10) reduce(+: x_associate)
end do
end associate
end subroutine
```
The declaration of `x_associate` is emitted as follows:
```mlir
%13:2 = hlfir.declare %10(%12) {uniq_name = "...."} : (!fir.heap<!fir.array<?xf32>>, !fir.shapeshift<1>) -> (!fir.box<!fir.array<?xf32>>, !fir.heap<!fir.array<?xf32>>)
```
where the HLFIR base type is an array descriptor (i.e. the
allocatable/heap attribute is dropped as stipulated by the spec; section
11.1.3.3).
The problem here is that `declare_reduction` ops accept only reference
types. This restriction is already partially handled for
`fir::BaseBoxType`'s by allocating a stack slot for the descriptor and
storing the box in that stack allocation. We have to modify this a
littble bit for `associate` since the HLFIR and FIR base types are
different (unlike most scenarios).
llvm-project
a716cc09 - [flang] Support `do concurrent ... reduce` for associating names (#148597)
