[clangd] Config: loading and caching config from disk.

Summary:
The Provider extension point is designed to also be implemented by
ClangdLSPServer (to inject config-over-lsp) and likely by embedders.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82964