The redundancy around work-done-progress is annoying but ok for now.
There's a weirdness with context lifetimes around outgoing method calls, which
I've preserved to keep this NFC. We should probably fix it though.
Differential Revision: https://reviews.llvm.org/D96717
The goal is to allow the LSP bindings of features to be defined outside
the ClangdLSPServer class, turning it into less of a monolith.
Differential Revision: https://reviews.llvm.org/D96544