[clangd] Add categories to help options, and only show clangd options.

[clangd] Add categories to help options, and only show clangd options.

Summary:
Restricting the categories prevents extra unwanted options from creeping into
help (D60663), and removes a bunch of noise from --help-hidden.

While here, remove `static` from the opts in favor of an anon namespace, to
reduce the noise level.

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65200

llvm-svn: 366900