[clangd] Send CodeAction responses to textDocument/codeAction (LSP 3.8)

Summary:
I don't bother mirroring the full capabilities struct, just parse the
bits we care about. I'll send a new patch to use this approach elsewhere too.

Reviewers: kadircet

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D53213

llvm-svn: 344617