more accurate omp in fortran

[Cesar Philippidis]

Currently, for certain omp and oacc errors the fortran will inaccurately
report exactly where in the omp/acc construct the error has occurred. E.g.

   !$acc parallel copy (i) copy (i) copy (j)
                                           1
Error: Symbol ‘i’ present on multiple clauses at (1)

instead of

   !$acc parallel copy (i) copy (i) copy (j)
                                1
Error: Symbol ‘i’ present on multiple clauses at (1)

The problem here is how the front end uses the locus for the construct
and not the individual clause. As a result that diagnostic pointer
points to the end of the construct.

This patch teaches gfc_resolve_omp_clauses how to use the locus of each
individual clause instead of the construct when reporting errors
involving OMP_LIST_ clauses (which are typically clauses involving
variables). It's still not perfect, but it does improve the quality of
the error reporting a little. In particular, in openacc, other compilers
are somewhat lenient in allowing variables to appear in multiple
clauses, e.g. copyin (foo) copyout (foo), but this is clearly forbidden
by the spec. I received some bug reports complaining that gfortran's
errors aren't accurate.

I've also split off the check for variables appearing in multiple
clauses into a separate function. It's a little overkill for trunk right
now, but it is used quite a bit in gomp4 for oacc declare.

I've tested these changes on x86_64. Is this ok for trunk?

Cesar


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