Currently, the `mlir-tblgen -verify-openmp-ops` pseudo-backend, which
only performs an OpenMP dialect-specific set of checks and produces no
output, is prevented from being added as a dependency to the
`MLIROpenMPOpsIncGen` tablegen target.
However, a consequence of this is that it is not triggered with every
modification of the OpenMPOps.td file it's intended to check, although
it should. This patch fixes the issue by letting the empty output file
to be added to the `TABLEGEN_OUTPUT` CMake variable used by the
`add_public_tablegen_target` command below to set up dependencies.
cba63c0f92