This replaces the (only) call to llvm::sys::fs::openFileForWrite with FileSystem::Open. This guarantees that we include log files in the reproducers.
This looks okay to me though I'm not very familiar with the llvm file system interfaces.
Do we have any tests that tests that log output gets emitted to the file requested when you do "log enable -f somefile whatever"? If so and they still work, LGTM. If we don't test that at all, we should probably add a test on the principle of "when you monkey with something that doesn't have a test, you should add one..."