[clangd] Config: Fragments and parsing from YAML

Summary:
This is a piece from the design in https://tinyurl.com/clangd-config
https://reviews.llvm.org/D82335 is a draft with all the bits linked together.
It doesn't do anything yet, it's just a library + tests.

It deliberately implements a minimal set of actual configuration options.

Reviewers: kadircet, adamcz

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82386