This patch updates the way that private reductions are handled in gomp4
to be more like trunk. Before, omp lower was using a complicated mapping
for private variables, but now it's treating them more like omp, with
the exception of reference-type variables. This complication originated
back when we were using ganglocal memory for private variables. Now that
private variables just regular local variables, I was able to remove a
lot of that old code.

It should be noted that reference-type variables still rely on
gimplifier creating a special OMP_CLAUSE_REDUCTION_PRIVATE_DECL, which
is basically a local copy of the reduction variable. Currently this is
used when the reduction variables are dummy arguments in fortran or
parallel (non-loop) reductions. I want to get rid of the
localize_reductions pass from the gimplifier eventually, but for the
time being this patch does fix pr/68813.

In the process of removing removing that old private code, I noticed
that lower_oacc_reductions couldn't handle reductions of the form

  #pragma acc loop reduction (+:v)
  for (...)
    #pragma acc loop reduction (+:v)
     for (...)

That's fixed now. In addition to teaching lower_oacc_reductions about
private variables, I also taught it how to update any intermediate
reduction variable when present. I'll port over this change to trunk
once I've resolved the localize_reductions issue in the gimplifier.

I don't have recent baseline, but I am seeing these failures:

  g++.sum:c-c++-common/goacc/routine-7.c
  libgomp.oacc-c/../libgomp.oacc-c-c++-common/declare-4.c

I'll work on routine-7.c tomorrow. Jim, can you look at the declare-4.c
failure?

This patch has been applied to gomp-4_0-branch.

Cesar