The strippable Swift reflection sections contain subtractor relocations
that need to be applied. There are two situations we need to support.
- Both symbols used in the relocation come from the .o file (for example, one symbol lives in swift5_fieldmd and the second in swift5_reflstr).
- One symbol comes from th .o file and the second from the main binary (for example, swift5_fieldmd and swift5_typeref).
The keys in this map will be integers from 0..~10. Should this just be a std::vector instead?