[flang][openacc] Disallow the zero modifier on the copyout clause with the declare directive

The OpenACC 3.3 specification does not allow the `zero` modifier
on the `copyout` clause used with the declare directive.

This is similar to D156703 for the create clause.

This might be missing piece in the spec but we disallow it until proven otherwise.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D156825