Hi,

this patch implements a warning about (some simple cases of direct)
modifications of iteration variables in OpenMP loops which are forbidden
according to the OpenMP specification. I think this can be helpful,
especially for new OpenMP users. I have implemented this after I
observed some confusion concerning this topic recently.
The check is implemented during gimplification. It reuses the
"loop_iter_var" vector in the "gimplify_omp_ctx" which was previously
only used for "doacross" handling to identify the loop iteration
variables during the gimplification of MODIFY_EXPRs in omp_for bodies.
I have only added a common C/C++ test because I don't see any special
C++ constructs for which a warning *should* be emitted and Fortran
rejects modifications of iteration variables in do loops in general.

I have run "make check" on x86_64-linux-gnu and not observed any
regressions.

Is it ok to commit this?

Best regards,
Frederik