[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