[mlir] Fix -Wunused-function in TransformInterpreterPassBase.cpp (NFC)

/llvm-project/mlir/lib/Dialect/Transform/Transforms/TransformInterpreterPassBase.cpp:167:1: error: unused function 'saveReproToTempFile' [-Werror,-Wunused-function]
saveReproToTempFile(llvm::raw_ostream &os, Operation *target,
^
1 error generated.