[clangd] Extend textDocument/didChange to specify whether diagnostics should be generated.

Summary:
This would allow us to disable diagnostics when didChange is called but
diagnostics are not wanted (e.g. code completion).

Reviewers: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43634

llvm-svn: 325813