[lld-macho] add tests for ICF, plus cleanups

Add tests for pending TODOs, plus some global cleanups:
* No fold: func has personality/LSDA
* Fold: reference to absolute symbol with different name but identical value
* No fold: reloc references to absolute symbols with different values
* No fold: N_ALT_ENTRY symbols

Differential Revision: https://reviews.llvm.org/D104721