Add hints with the values of constants to the popup menu

I'm not quite satisfied with the UI for this.  Suggestions for improvements appreciated.
This should be tested more thoroughly than your average patch.  VarDecl::evaluateValue tends to crash if you call it without all of its implicit preconditions being satisfied.  I think I have fixed these all now but there might be some still lurking.
This is built on top of previous PRs.  I can split it out if need be.