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.