Reenable optimised builds when compiling from didChange events.

[JoshuaBatty]

After solving a bug in #6575 that was crashing the server on did save events, I noticed that we do not return any diagnostics to the user.

Iv'e played around for a while but it's not obvious to me the best way to solve this. Considering we have a code freeze coming up i'm temporarily disabling this optimisation so we can continue to show error and warning diagnostics to the user while I investigate.