The problem was caused by a race condition of all things. ClangCompleter would
set possibly_completions_ready when starting the first parse pass for the file
and then would try to extract diagnostics for the file before the diagnostics
were done. Technically this was not a problem because only an empty diagnostics
vector would be returned, but this triggered Syntastic because hey, we have some
diagnostics to show (even though we don't).
And then Syntastic would try to close the location list window during startup
when this operation is not available. Technically it's Syntastic's fault, but a
more principled way to check for done diagnostics is to return and use a future
for file parsing operations and this solution also works around the Syntastic
issue.
f99e89f812