[lld-macho] Support folding of functions with identical LSDAs

To do this, we need to slice away the LSDA pointer, just like we are
slicing away the functionAddress pointer.

No observable difference in perf on chromium_framework:

             base           diff           difference (95% CI)
  sys_time   1.769 ± 0.068  1.761 ± 0.065  [  -2.7% ..   +1.8%]
  user_time  9.517 ± 0.110  9.528 ± 0.116  [  -0.6% ..   +0.8%]
  wall_time  8.291 ± 0.174  8.307 ± 0.183  [  -1.1% ..   +1.5%]
  samples    21             25

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D129830