[lld-macho] Support renaming of LSDA section

Previously, our unwind info finalization logic assumed that the LSDA
section referenced by `__compact_unwind` was already finalized before
`__TEXT,__unwind_info` itself. However, that assumption could be broken
by the use of `-rename_section` -- it could be (and is) used to move
`__gcc_except_tab` it into a different segment later in the file.
(__TEXT is always the first non-zerofill segment, so any rename
basically guarantees that the section will be ordered after
`__unwind_info`.)

To handle this case, we compare LSDA relocations instead of their final
values in `UnwindInfoSection::finalize()`, and we actually relocate
those LSDAs in `UnwindInfoSection::writeTo()`. In order to do this, we
need an easy way to track which Symbol a given CUE corresponds to. My
solution was to change our `cuPtrVector` into a vector of indices, with
each index used for both the symbols vector (`symbolsVec`) as well as
the CUE vector (`cuVector`).

This change seems perf neutral. Numbers for linking chromium_framework
on my 16 core Mac Pro:

             base           diff           difference (95% CI)
  sys_time   1.248 ± 0.025  1.245 ± 0.026  [  -1.3% ..   +0.8%]
  user_time  3.588 ± 0.045  3.587 ± 0.037  [  -0.6% ..   +0.5%]
  wall_time  4.605 ± 0.069  4.595 ± 0.069  [  -1.0% ..   +0.5%]
  samples    42             26

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D113582