[clangd] Config: config struct propagated through Context

Summary:
This introduces the "semantic form" of config exposed to features,
contrasted with the "syntactic form" exposed to users in e9fb1506.

The two are not connected, CompiledFragment and Provider will bridge that gap.
Nor is configuration actually set: that needs changes to ClangdServer,
TUScheduler, and BackgroundQueue.

Reviewers: hokein, kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82606