[gomp4] fix c++ reference mappings in openacc

[Cesar Philippidis]

I've applied this patch to gomp-4_0-branch which fixes of problems
involving reference type variables in openacc data clauses. The first
problem was, the c++ front end was incorrectly handling reference types
in general in openacc. Instead of mapping the variable, it would map the
pointer to the variable by itself. The second problem was, if the
gimplifier saw a pointer mapping for a data clause, it would propagate
it to omp-lower. That's bad because if you have something like this

  int &var = ...

  #pragma acc data copy (var)
  {
     ...var...
  }

where the var inside the data region would have some uninitialized value
because omplower installs a new variable for it. The gimpifier is
already handling openmp target data regions properly, so this patch
extends it to ignore pointer mappings in acc enter/exit and data constructs.

Ultimately this patch will need to go in trunk, but the c++ changes
don't apply cleanly. I'll need to work on that later.

Cesar