Natvis, CV8, dAssocArray

[Void-995]

I almost finished my -gc for Natvis experiment (to use MS C++ 
Debugger from VS Code, fully translating type names from what 
comes from DMD frontend to valid C++ type name so Natvis works), 
but then I encountered dAssocArray, which has form of: void* as 
CodeView structure member, KeyType __key_t and ValueType __val_t 
as nested types.

As for someone who just found about CodeView today: what is 
nested type, what is the deal with void*, what to do with __key_t 
and __val_t to get values.