[mlir][flang] Fix -Wunused-result after D146785 (NFC)
/data/llvm-project/mlir/examples/toy/Ch4/toyc.cpp:119:5: error: ignoring return value of function declared with 'nodiscard' attribute [-Werror,-Wunused-result]
applyPassManagerCLOptions(pm);
^~~~~~~~~~~~~~~~~~~~~~~~~ ~~
1 error generated.
/data/llvm-project/flang/lib/Frontend/FrontendActions.cpp:669:3: error: ignoring return value of function declared with 'nodiscard' attribute [-Werror,-Wunused-result]
mlir::applyPassManagerCLOptions(pm);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~
1 error generated.