[clangd] Send suppported codeActionKinds to the client.

Summary:
This would make client know which codeActionKinds that clangd may
return.

VSCode will add a new entry "Refactor..." (which shows all
refactoring-kind code actions) in the right-click menu.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66592

llvm-svn: 369656