[clang] Move SPELLING_CACHE into CodeCompletion (PR #177586)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Can you commit reordering of `CompletionChunk` and `CompletionChunkKind` to `main` as NFC? This is hard to review as it is.

https://github.com/llvm/llvm-project/pull/177586