[dsymutil] Apply relocations present in Swift reflection sections

The strippable Swift reflection sections contain subtractor relocations
that need to be applied. There are two situations we need to support.
1) 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).
2) One symbol comes from th .o file and the second from the main
   binary (for example, __swift5_fieldmd and __swift5_typeref).

Differential Revision: https://reviews.llvm.org/D120574