[lld] [lld-macho][ObjC] Implement category merging into base class (PR #92448)

[Ellis Hoag via llvm-commits]

================
@@ -552,6 +556,32 @@ ObjcCategoryMerger::tryGetDefinedAtIsecOffset(const ConcatInputSection *isec,
   return dyn_cast_or_null<Defined>(sym);
 }
 
+// Get the class's ro_data symbol. If getMetaRo is true, then we will return
+// the meta-class's ro_data symbol. Otherwise, we will return the class
+// (instance) ro_data symbol.
+Defined *ObjcCategoryMerger::getClassRo(const Defined *classSym,
+                                        bool getMetaRo) {
+  ConcatInputSection *isec = dyn_cast<ConcatInputSection>(classSym->isec());
+  if (!isec)
+    return nullptr;
+
+  Defined *classRo = nullptr;
+  if (getMetaRo) {
+    Defined *metaClass = tryGetDefinedAtIsecOffset(
+        isec, classLayout.metaClassOffset + classSym->value);
+
+    classRo = metaClass ? tryGetDefinedAtIsecOffset(
+                              dyn_cast<ConcatInputSection>(metaClass->isec()),
+                              classLayout.roDataOffset)
+                        : nullptr;
+  } else {
+    classRo = tryGetDefinedAtIsecOffset(isec, classLayout.roDataOffset +
+                                                  classSym->value);
+  }
+
+  return classRo;
----------------
ellishg wrote:

I think this should be equivalent.

```suggestion
  if (!getMetaRo)
    return tryGetDefinedAtIsecOffset(isec, classLayout.roDataOffset + classSym->value);
  
  Defined *metaClass = tryGetDefinedAtIsecOffset(isec, classLayout.metaClassOffset + classSym->value);
  if (!metaClass)
    return nullptr;

  return tryGetDefinedAtIsecOffset(dyn_cast<ConcatInputSection>(metaClass->isec()), classLayout.roDataOffset);
```

https://github.com/llvm/llvm-project/pull/92448