[llvm] [DevPolicy] Add guidance to not submit code or reviews on someone else's (PR #69701)

[Alex Bradbury via llvm-commits]

asb wrote:

Very minor: please undo the truncation of the commit title when committing (I documented this gotcha [here](https://llvm.org/docs/GitHub.html#creating-pull-requests) after finding my experience skimming through commits worsened due to this annoying GitHub behaviour.

I agree with the points raised by @efriedma-quic, especially that the guidance seems a bit mysterious and hard to interpret without the context of the announcement that went out. Perhaps adding something like "If you have any suspicion that someone is asking you to submit work for them in order to evade a ban, please contact the CoC team.".

I also note there are other parts of the developer policy that overlap to some degree, and I'm not sure if they should be updated as well:
* [Attribution of changes](https://llvm.org/docs/DeveloperPolicy.html#attribution-of-changes), which currently says "If someone sends you a patch privately, encourage them to submit it to the appropriate list first."
* Some discussion under [commit messages](https://llvm.org/docs/DeveloperPolicy.html#commit-messages) on use of the co-authored-by tag.

https://github.com/llvm/llvm-project/pull/69701