[gomp4] remap variables inside gang, worker, vector and collapse clauses

[Cesar Philippidis]

Gang, worker, vector and collapse all contain optional arguments which
may be used during loop expansion. In OpenACC, those expressions could
contain variables, but those variables aren't always getting remapped
automatically. This patch remaps those variables inside lower_omp_loop.

Note that I didn't need to use a tree walker for more complicated
expressions because it's not required. By the time those clauses reach
lower_omp_loop, only the result of the expression is available. So the
other variables in those expressions get remapped with everything else
during omplow. Therefore, the only problematic case is when the the
optional expression is just a decl, e.g. gang(static:foo).

I've applied this patch to gomp-4_0-branch.

Cesar