Make sure to refresh diagnostic UI on InsertLeave

In case the current filetype language server uses LSP async diagnostics,
we might not get a reparse on InsertLeave. This happens if the user
leaves insert mode after a semantic trigger.

We would still try sending a FileReadyToParse request, but the diags get
ignored, because:

a) The filetype is known to use async diags.
b) We do not insist on a synchronous diag update, like :YcmDiags.

The solution is to check if the current filetype uses async diagnostics
and, if so, let InsertLeave refresh diagnostic UI regardless of what the
state of FileReadyToParse request is.

Note that this solution makes the flicker on leaving insert mode harder
to fix. Not only does OnCursorMoved cause a refresh of stale
diagnostics, but so does OnInsertLeave.

python/ycm/youcompleteme.py

@@ -614,8 +614,10 @@ def OnInsertEnter( self ):
 
 
   def OnInsertLeave( self ):
+    async_diags = any( self._message_poll_requests.get( filetype )
+                      for filetype in vimsupport.CurrentFiletypes() )
     if ( not self._user_options[ 'update_diagnostics_in_insert_mode' ] and
-         not self.CurrentBuffer().ParseRequestPending() ):
+         ( async_diags or not self.CurrentBuffer().ParseRequestPending() ) ):
       self.CurrentBuffer().RefreshDiagnosticsUI()
     SendEventNotificationAsync( 'InsertLeave' )