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

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.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D97576

lld/MachO/InputFiles.cpp

@@ -330,8 +330,7 @@ void ObjFile::parseRelocations(const section_64 &sec,
         // 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 {