IRBuilder: ensure names, entity header, consts.
Ensure that "structs" have names, so that structs can be referred symbolically. In the future, functions and basic blocks also need to be ensured to have names. Such names are generated from their IDs if not provided by the client. The IR builder can create named or unnamed MuEntityHeader based on whether it has client-provided name or not. Can build constants now (currently only Constant::Int).