Fixes #149089 and #149700.

Before #145837, when processing a reduction symbol not yet supported by
OpenMP lowering, the reduction processor would simply skip filling in
the reduction symbols and variables. With #145837, this behvaior was
slightly changed because the reduction symbols are populated before
invoking the reduction processor (this is more convenient to shared the
code with `do concurrent`).

This PR restores the previous behavior.

See https://github.com/llvm/llvm-project/pull/147168 for more info.

GCC 9.3.0
```
.../flang/lib/Lower/Support/ReductionProcessor.cpp:137:1: error: control reaches end of non-void function [-Werror=return-type]
```

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).

Handles some loose ends in `do concurrent` reduction declarations. This
PR extends `getAllocaBlock` to handle declare ops, and also emit
`fir.yield` in all regions.

With #145837, the `ReductionProcessor` component is now used by both
OpenMP and `do concurrent`. Therefore, this PR moves it to a shared
location: `flang/Lower/Support`.

PR stack:
- https://github.com/llvm/llvm-project/pull/145837
- https://github.com/llvm/llvm-project/pull/146025 (this one)
- https://github.com/llvm/llvm-project/pull/146028
- https://github.com/llvm/llvm-project/pull/146033