This change is the next step in unification of text synchronization
methods. The logic really belongs in the internal/lsp/cache package
rather than the internal/lsp package.
Pulled out a function to run diagnostics on a file (diagnostics are still
run async).
Change-Id: I5e237411a02af210ad386b37a6c2aa62ef723567
Reviewed-on: https://go-review.googlesource.com/c/tools/+/210784
Reviewed-by: Heschi Kreinick <heschi@google.com>