[lld/mac] Add some comments and asserts

[thakis]

Summary

I was wondering if SymtabSection::emitStabs() should check
defined->includeInSymtab. Add asserts and comments explaining why that's not
necessary.

No behavior change.

[oontvoo]

thanks!

lld/MachO/SymbolTable.cpp

Symbol *SymbolTable::addLazyObject(StringRef name, InputFile &file) {
   }
   return s;
 }
 
 Defined *SymbolTable::addSynthetic(StringRef name, InputSection *isec,
                                    uint64_t value, bool isPrivateExtern,
                                    bool includeInSymtab,
                                    bool referencedDynamically) {
+  assert(!isec || !isec->getFile()); // See makeSyntheticInputSection().
   Defined *s =
-      addDefined(name, nullptr, isec, value, /*size=*/0,
+      addDefined(name, /*file=*/nullptr, isec, value, /*size=*/0,
                  /*isWeakDef=*/false, isPrivateExtern,
                  /*isThumb=*/false, referencedDynamically,
                  /*noDeadStrip=*/false, /*isWeakDefCanBeHidden=*/false);
   s->includeInSymtab = includeInSymtab;
   return s;
 }
 
 enum class Boundary {

lld/MachO/SyntheticSections.cpp

if (auto *defined = dyn_cast<Defined>(sym)) {
       // get a stabs entry.
       if (defined->wasIdenticalCodeFolded)
         continue;
 
       InputSection *isec = defined->isec;
       ObjFile *file = dyn_cast_or_null<ObjFile>(isec->getFile());
       if (!file || !file->compileUnit)
         continue;
+
+      // All symbols that set includeInSymtab to false are synthetic symbols.
+      // Those have `file` set to nullptr and were already skipped due to that.
+      assert(defined->includeInSymtab);
+
       symbolsNeedingStabs.push_back(defined);
     }
   }
 
   llvm::stable_sort(symbolsNeedingStabs, [&](Defined *a, Defined *b) {
     return a->isec->getFile()->id < b->isec->getFile()->id;
   });