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

[Krzysztof Parzyszek via cfe-commits]

kparzysz wrote:

Here's another thing---could there be tools that try to parse the messages (e.g. something that runs clang and presents the messages to the user in some form)?  Having a policy such as "single quotes only come in pairs" could make it easier.  I don't know if that's something we should even take into consideration, it's just a thought.

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