Style updates, mostly _ naming (#970)

There are some declaration order changes, and a few test classes switched from `struct` to `class`. However, this PR is mostly adopting `_` naming of private member variables due to the shift in naming style. None of what's here should have behavior impacts, it should just be style.

Note, there are a lot of things that *look* like they could be accessor-named, but I'm not doing that in this change. Happy to do it separately if you want me to do another PR focused on it.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/parser/parse_node_kind.cpp

@@ -13,7 +13,7 @@ auto ParseNodeKind::GetName() const -> llvm::StringRef {
 #define CARBON_PARSE_NODE_KIND(Name) #Name,
 #include "toolchain/parser/parse_node_kind.def"
   };
-  return Names[static_cast<int>(kind)];
+  return Names[static_cast<int>(kind_)];
 }
 
 }  // namespace Carbon