[clangd] Delay binding LSP methods until initialize. NFC

This is NFC because the MessageHandler refused to dispatch to them until the
server is initialized anyway.

This is a more natural time to bind them - it's when they become callable, and
it's when client capabalities are available and server ones can be set.

One module-lifecycle function will be responsible for all three.

Differential Revision: https://reviews.llvm.org/D96608