Add a MemberName parse node category for member access expressions (#3549)

Use this new category to replace the unconstrained `NodeId` child of
`MemberAccessExpr ` and `PointerMemberAccessExpr`. For now this new
category matches `IdentifierName` and `BaseName`, but later this will be
expanded to support `a.(b.c)` and `p->(b.c)` syntactic forms.

QUESTION: Is it time to make a `node_category.def` x-macro file?
ANSWER: Not yet.

toolchain/parse/node_kind.cpp

@@ -23,6 +23,7 @@ auto operator<<(llvm::raw_ostream& output, NodeCategory category)
   }
     CARBON_NODE_CATEGORY(Decl);
     CARBON_NODE_CATEGORY(Expr);
+    CARBON_NODE_CATEGORY(MemberName);
     CARBON_NODE_CATEGORY(Modifier);
     CARBON_NODE_CATEGORY(NameComponent);
     CARBON_NODE_CATEGORY(Pattern);