Use LLVM casting for TypeSystem so you can cast it to subclasses.

This will keep our code cleaner and it removes the need for intrusive additions to TypeSystem like:

class TypeSystem
{
    virtual ClangASTContext *
    AsClangASTContext() = 0;
}

As you can now just use the llvm::dyn_cast and other casts.

llvm-svn: 247041