Once an unsubstituted typeref for a field is built, we substitute
`GenericTypeParameterTypeRef`s with concrete ones built from the generic
arguments of concrete bound generic metadata.
During that process, if we run into a `DependentMemberTypeRef` (e.g.
something of type T.Index), we substitute the base (T) using the current
list of substitutions, and then resolve what `Index` is for the base
using the associated type metadata in the 'assocty' data section.
These likely don't have Swift type metadata but might be useful to
record or instantiate based on what kind of metadata is encountered for
the sake of memory tools.