[clangd] Embed fixes as CodeAction, instead of clangd_fixes. Clean up serialization.

Summary:
CodeAction provides us with a standard way of representing fixes inline, so
use it, replacing our existing ad-hoc extension.

After this, it's easy to serialize diagnostics using the structured
toJSON/Protocol.h mechanism rather than assembling JSON ad-hoc.

Reviewers: hokein, arphaman

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

Differential Revision: https://reviews.llvm.org/D53391

llvm-svn: 345119