Move empty atom check to target independent code. NFC.

Turns out that checking only x86 for empty atoms to fix UBSan then
requires the same code in the other targets too.  Better to just
check this in the main run loop instead of in each target.

Should be NFC, other than fixing UBSan failures.

llvm-svn: 264116

lld/lib/ReaderWriter/MachO/ArchHandler_x86.cpp

@@ -420,11 +420,6 @@ void ArchHandler_x86::generateAtomContent(const DefinedAtom &atom,
                                           FindAddressForAtom findSectionAddress,
                                           uint64_t imageBaseAddress,
                                           uint8_t *atomContentBuffer) {
-  if (!atom.size()) {
-    assert(atom.begin() == atom.end() &&
-           "Cannot have references without content");
-    return;
-  }
   // Copy raw bytes.
   memcpy(atomContentBuffer, atom.rawContent().data(), atom.size());
   // Apply fix-ups.

lld/lib/ReaderWriter/MachO/MachONormalizedFileFromAtoms.cpp

@@ -672,6 +672,11 @@ void Util::copySectionContent(NormalizedFile &file) {
     uint8_t *sectionContent = file.ownedAllocations.Allocate<uint8_t>(si->size);
     normSect->content = llvm::makeArrayRef(sectionContent, si->size);
     for (AtomInfo &ai : si->atomsAndOffsets) {
+      if (!ai.atom->size()) {
+        assert(ai.atom->begin() == ai.atom->end() &&
+               "Cannot have references without content");
+        continue;
+      }
       uint8_t *atomContent = reinterpret_cast<uint8_t*>
                                           (&sectionContent[ai.offsetInSection]);
       _archHandler.generateAtomContent(*ai.atom, r, addrForAtom,