[lld-macho] Canonicalize LSDA pointers

This was causing an uncaught exception issue in one of our programs. The
issue was fairly subtle / rare as it required two identical LSDAs that were
referenced by a pair of non-identical compact unwind encodings.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D139269