Add script and test to check for any unused Diagnostics in the codebase

[mininny]

This PR adds a python script and swift test file to check for any unused diagnostics in the codebase.
If uses regex to get all the diagnostics kinds from Diagnostic~.def files, and uses aho-corasick algorithm for faster searching of all the diagnostics in all files under /lib

• I'm not yet sure how to properly use pythong script inside a swift test.
• I'm not sure if it's okay to use external pip package aho-corasick inside swift test - would it be better to write the implementation in the python script entirely?

Resolves #67306

[tbkka]

@hamishknight @hborla I wonder if this script should be used in a test? That would help ensure we don't inadvertently leave behind unused diagnostics.

[mininny]

Hi @hamishknight @hborla,
I'd appreciate it if you could give any input about this script.

If you believe it's not suitable for adding to a test, I can document the stale diagnostics I've identified and create the necessary issues for their removal.

Thanks :)

[hamishknight]

I think you may need to check ./include too since we could emit diagnostics in headers

[hamishknight]

Sorry for the late reply! Adding it as a test seems reasonable to me, cc @xedin too. The script looks good to me modulo the above comment. Regarding using an external pip package, I believe we already have a python script test that depends on an external pip package (test/Python/python_lint.swift), though I think ideally we'd keep the dependencies to a minimum. Out of curiosity, how much longer does the script take doing a standard regex search?