[openacc] fortran loop clauses and splitting

[Cesar Philippidis]

This patch contains the following bug fixes:

 * Teaches gfortran to accept both num and static gang arguments inside
   same clause. E.g. gang(num:10, static:30). Currently, gfortran only
   allows one of those arguments to appear in a gang clause.

 * Make the diagnostics reported by resovle_oacc_positive_int_expr more
   accurate for worker and vector clauses.

 * Updates how combined loops are split to account for the renamed gang
   clause members in gfc_omp_clauses.  Also corrected a bug that Tom
   discovered in the c front end where combined reductions were being
   attached to kernels and parallel constructs. Now, they are only
   associated with the split acc loop.

Is this OK for trunk?

Cesar
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