[clangd] Cancel certain operations if the file changes before we start.

Summary:
Otherwise they can force us to build lots of snapshots that we don't need.
Particularly, try to do this for operations that are frequently
generated by editors without explicit user interaction, and where
editing the file makes the result less useful. (Code action
enumeration is a good example).

https://github.com/clangd/clangd/issues/298

This doesn't return the "right" LSP error code (ContentModified) to the client,
we need to teach the cancellation API to distinguish between different causes.

Reviewers: kadircet

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, jfb, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75602