[lld-macho] ICF: Do more work in equalsConstant, less in equalsVariable

In particular, relocations to absolute symbols or literal sections can
be handled in equalsConstant(), since their output addresses will not
change across each iteration of ICF. Offsets and addends can also be
dealt with entirely in equalsConstant(), making the code somewhat easier
to reason about. Only ConcatInputSections need to be handled in
equalsVariable().

LLD-ELF's implementation takes a similar approach.

Although this should make ICF do less work, in practice it seems like
there is no stat sig difference in time taken when linking
chromium_framework.

This refactor is motivated by an upcoming diff which improves ICF's handling of
addends.

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D106212