swift234

[Reflection] Substitute generics and resolve dependent members

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.