[clangd] Send the correct error code when cancelling requests.

Summary:
I couldn't quite bring myself to make Cancellation depend on LSP ErrorCode.
Magic numbers instead...

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77947