[clangd] Add library to semantically strip flags by name.

Summary:
This is designed for tweaking compile commands by specifying flags to add/remove
in a config file. Something like:
  CompileFlags: { Remove: -fcolor-diagnostics }

Having users tweak raw argv (e.g. with a regex) is going to end in tears: bugs
around clang-cl, xclang, aliases, joined-vs-separate args etc are inevitable.

This isn't in tooling because of the performance choices: build a big table
up-front to make subsequent actions fast. Maybe it should be though.

Reviewers: adamcz, hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81958