The move-analysis portion of borrowck needs to work in an abstract
domain of lifted Places. Most of the Place variants fall into a
one-to-one mapping between the concrete and abstract (e.g., a
field-deref on a local variable, x.field, has the same meaning
in both domains). Indexed projections are the exception: a[x]
needs to be treated as mapping to the same move path as a[y] as
well as a[13], etc. So we map these x/y values to ().