Keep output file after successful execution of mlir-opt

Invoke `keep()` on the output file of `mlir-opt` in case the invocation of `MlirOptMain` was successful, to make sure the output file is not deleted on exit from `mlir-opt`.
Fixes a similar problem in `standalone-opt` from the example for an out-of-tree, standalone MLIR dialect.

This revision also adds a missing parameter to the invocation of `MlirOptMain` in `standalone-opt`.

Differential Revision: https://reviews.llvm.org/D77643