NLL Diagnostic Review 3: missing borrow error justifications for unions

For the `ui/borrowck/borrowck-union-borrow.ast.nll.stderr` test, the AST borrow checker indicates that the borrow of `u.b` is causing the borrow of the union `u` itself, thus justifying the complaint about the borrow of `u.a`. 

However, the NLL borrow checker just says that `u.b` is already borrowed, which is less clear when its pointing at a borrow of `u.a`.