[lld-macho] Fix ICF crash when comparing symbol relocs

Previously, when encountering a symbol reloc located in a literal section, we
would look up the contents of the literal at the `symbol value + addend` offset
within the literal section. However, it seems that this offset is not guaranteed
to be valid. Instead, we should use just the symbol value to retrieve the
literal's contents, and compare the addend values separately. ld64 seems to do
this.

Reviewed By: #lld-macho, thevinster

Differential Revision: https://reviews.llvm.org/D124223