[clangd] Add hidden tweaks to dump AST/selection.

Summary:
This introduces a few new concepts:
 - tweaks have an Intent (they don't all advertise as refactorings)
 - tweaks may produce messages (for ShowMessage notification). Generalized
   Replacements -> Effect.
 - tweaks (and other features) may be hidden (clangd -hidden-features flag).
   We may choose to promote these one day. I'm not sure they're worth their own
   feature flags though.

Verified it in vim-clangd (not yet open source), curious if the UI is ok in VSCode.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62538

llvm-svn: 363680