[clangd] Don't cancel requests based on "updates" with same content
There's an unfortunate collision between two features:
- we implicitly cancel certain requests when the file changes, to avoid
the queue getting clogged building old revisions to service stale requests
- we "reparse-if-needed" by synthesizing a file change, e.g. on didSave
We could explicitly mark these synthetic requests to avoid this, but
looking for changes in file content clutters our APIs less and is
arguably the correct thing to do in any case.
Fixes https://github.com/clangd/clangd/issues/620