[MLIR][OpenMP] Keep -verify-openmp-ops output as dependency (llvm#99638)

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.

mlir/include/mlir/Dialect/OpenMP/CMakeLists.txt

@@ -4,14 +4,14 @@ add_public_tablegen_target(omp_common_td)
 
 set(LLVM_TARGET_DEFINITIONS OpenMPOps.td)
 
-# Run the OpenMP verifier tablegen pseudo-backend while preventing the produced
-# dummy output from being added as a dependency to any tablegen targets defined
-# below.
-set(TABLEGEN_OUTPUT_TMP ${TABLEGEN_OUTPUT})
+# The OpenMP verifier tablegen pseudo-backend does not produce any output, but
+# mlir_tablegen expects an output file name to be passed. An empty "no-output"
+# file is created by the statement below.
+#
+# This output will be added to the list of dependencies of the
+# MLIROpenMPOpsIncGen target below, which results in triggering this
+# verification pass every time OpenMPOps.td is modified and recompiled.
 mlir_tablegen(no-output -verify-openmp-ops)
-file(REMOVE ${CMAKE_CURRENT_BINARY_DIR}/no-output ${CMAKE_CURRENT_BINARY_DIR}/no-output.d)
-set(TABLEGEN_OUTPUT ${TABLEGEN_OUTPUT_TMP})
-unset(TABLEGEN_OUTPUT_TMP)
 
 mlir_tablegen(OpenMPOpsDialect.h.inc -gen-dialect-decls -dialect=omp)
 mlir_tablegen(OpenMPOpsDialect.cpp.inc -gen-dialect-defs -dialect=omp)