[process-stats-dir] Fix a couple of issues

[hamishknight]

• Fix an error that was produced when specifying --output, as the FileType must be buffered for a non-binary output.
• Fix an indexing error, as it seems namedtuple no longer supports vars(...)

[edymtt]

Looks good to me -- please consider generating toolchains and/or testing the compiler performance presets to test the program (what I should have done in the first place for #58755)

[edymtt]

Thanks for taking the time to factor this in a separate function!
This makes the intent of the below change easy to grasp.

[hamishknight]

@edymtt Unfortunately the compiler performance bots currently aren't working, I have tested it locally against some CI-generated stats dirs though. By toolchain do you mean that please build toolchain will run the script? Happy to try that

[etcwilde]

Mostly lint-nit cleanups.
process will need the parser passed in to handle printing the help message. Alternatively, the parser.print_help() could probably be pulled back into main.

Otherwise LGTM.

[hamishknight]

@swift-ci please smoke test

[edymtt]

@hamishknight got it, local tests will do for now
yes, that's the command I had in mind, as per https://github.com/apple/swift/blob/main/docs/ContinuousIntegration.md#build-swift-toolchain