[lld-macho] Handle TAPI and regular re-exports uniformly

The re-exports list in a TAPI document can either refer to other inlined
TAPI documents, or to on-disk files (which may themselves be TBD or
regular files.) Similarly, the re-exports of a regular dylib can refer
to a TBD file.

Differential Revision: https://reviews.llvm.org/D85404