What does the Visitor class do in DMD exactly?

[solidstate1991]

Currently I'm in the process of adding address space pointer 
support to LDC in general (through a magic UDA as opposed to 
DCompute's magic struct), and so far I figured out I need to hack 
it in at the class `ToElemVisitor` and `DValue`, maybe also touch 
the files tollvm.cpp and irtype.cpp.

LDC has some issue with documentation, but will try to fix that 
too somewhat, at least with code that I have to touch.

Also what does the non-namespaced `Type` stand for? Because it 
has the same name as `llvm::Type`, more confusingly sometime by 
each other's side in the same function's argument list.