[clang] Clarify use of contractions in diagnostic messages (PR #116803)

[via cfe-commits]

Sirraide wrote:

> could there be tools that try to parse the messages

Hmm, I think we have other formats that are better suited for that (don’t we have a flag that makes us print JSON diagnostics?), so I’d *hope* that no-one tries to just parse the diagnostics from the terminal, and even then, you could definitely hard-code common contractions imo, but that is an interesting question nonetheless.

https://github.com/llvm/llvm-project/pull/116803