Optimisation: Do not cache source info in attributes when source annotation is not enabled

[craigfowler]

After the profiling work performed in #98, it has been noted that the caching of source annotation information in the attributes is prohibitively expensive and should not occur if source annotation is not enabled.

This functionality should be disabled when source annotation is switched off.

[craigfowler]

Interestingly, re-reading the profile report, it seems like the source annotation burn-in visitor is not the only thing which causes the source information to be 'burnt into' the elements.

It seems it's called from elsewhere also, in which case those other places also would be suppressed.