[DDMD] [refactor] Move Expression::toDt into a visitor class

Getting rid of `ClassReferenceExp::toDt` unfortunately means touching `ctfe.h`.

@WalterBright @donc Can we start breaking D1 compatibility in `ctfe.h`/`interpret.c`/`ctfeexpr.c` yet?  Switching to D will mean breaking that for sure, and we're quite close to that point now.

I'm going to need to visitor-ise `*::interpret` and `*::ctfeCompile` in the near future, probably in the next few days.