[WIP] Add support for code action quick fixes

[danielmartin]

[Still WIP]

Screenshot (VSCode):

This PR adds support for the textDocument/codeAction LSP method, so that Swift diagnostics that carry a quick fix can be applied from the LSP client with a keystroke.

[benlangmuir]

Very cool! One of the concerns I've had about using the codeAction API for fixits has been that the editor doesn't have a great way to keep the fixits in sync with the live diagnostics. Have you run into any synchronization issues testing this with vscode, or is it okay in practice?

[danielmartin]

I have yet to filter the available diagnostic fixits so that the VSCode light bulb only offers those that are relevant to where the cursor is, not every fixit for the current file, but it's looking good so far.

I think it would also make the interface with clangd simpler, as clangd is also using codeActions for quick fixes. Refactor actions also use this method, with the difference that, in the case of refactors, there's an extra step where the client sends the command to be executed to the server.

I had a previous implementation where the fixits simply came with the diagnostics in publishDiagnostics and that worked beautifully in Emacs but it didn't show a light bulb in VSCode. AFAIK that MS UI requires that the server answers to codeAction calls.

[benlangmuir]

@danielmartin FWIW we are planning to add the fixits to publishDiagnostics as well - see https://reviews.llvm.org/D50415 for clangd's implementation of that.

[cgarciae]

I'd love to see this feature become a reality!
Would be really useful for implementing protocols, right now I have to guess the correct functions.

[benlangmuir]

I believe this PR is obseleted by the combination of
#70
#216
#238

If there's something I missed, feel free to reopen after rebasing.