[lld-macho][nfc] Remove TODO regarding addends

[int3]

Summary

There was initially some concern around the correct handling of pcrel
section relocations with r_length != 2. But it looks like there are no such
relocations in practice -- x86_64's pcrel section relocs all have r_length == 2,
and ARM64 doesn't even have pcrel section relocs. So we can replace the TODO
with an assert.

lld/MachO/InputFiles.cpp

for (size_t i = 0; i < relInfos.size(); i++) {
     } else {
       SubsectionMap &referentSubsecMap = subsections[relInfo.r_symbolnum - 1];
       const section_64 &referentSec = sectionHeaders[relInfo.r_symbolnum - 1];
       uint32_t referentOffset;
       if (relInfo.r_pcrel) {
         // The implicit addend for pcrel section relocations is the pcrel offset
         // in terms of the addresses in the input file. Here we adjust it so
         // that it describes the offset from the start of the referent section.
-        // TODO: The offset of 4 is probably not right for ARM64, nor for
-        //       relocations with r_length != 2.
+        assert(target->hasAttr(r.type, RelocAttrBits::BYTE4));
         referentOffset =
             sec.addr + relInfo.r_address + 4 + totalAddend - referentSec.addr;
       } else {
         // The addend for a non-pcrel relocation is its absolute address.
         referentOffset = totalAddend - referentSec.addr;
       }
       r.referent = findContainingSubsection(referentSubsecMap, &referentOffset);
       r.addend = referentOffset;