Show members of DeclContexts (i.e., class members) in Visual Studio native visualizers

This change shows members of DeclContext objects in the Visual Studio debugger. It will also cast a TagType like a class or a struct to a DeclContext, so its methods and fields are visualized. 



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263794 91177308-0d34-0410-b5e6-96231b3b80d8