[clang] [Clang][Sema] Diagnose invalid 'pure'/'const' attributes on writes (PR #217817)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

@AaronBallman do you think this is something we should invest time in -- it's useful in some cases, but in other cases the writes may actually be semantically safe.

Change requests are all minor rather than anything major

https://github.com/llvm/llvm-project/pull/217817