[llvm] [llvm][nfc] Ignore OpenAI Codex artifacts (PR #162481)

[Peter Waller via llvm-commits]

https://github.com/peterwaller-arm approved this pull request.

For what it's worth, I'd tend to put this in my global git excludes rather than in each repo I work on. But I see that other models have already got a gitignore there - I don't (currently) see a reason to allow some but not others. Note that since you posted this your commit became conflicted becuase `/.gemini/` got generalised to `.gemini/` - maybe you could do the same?

https://github.com/llvm/llvm-project/pull/162481