[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
e9fb1506b83d.
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