[lld] [lld-macho][NFC] Refactor ObjCSelRefsSection out of ObjCStubsSection (PR #83878)

[Kyungwoo Lee via llvm-commits]

================
@@ -315,6 +315,25 @@ class StubHelperSection final : public SyntheticSection {
   Defined *dyldPrivate = nullptr;
 };
 
+class ObjCSelRefsSection final : public SyntheticSection {
+public:
+  ObjCSelRefsSection();
+  void initialize();
+
+  // This SyntheticSection does not do its own writing, but instead uses the
+  // creates SyntheticInputSection's and inserts them into inputSections
+  uint64_t getSize() const override { return 0; }
----------------
kyulee-com wrote:

It's odd to inherit `ObjCSelRefsSection` from `SyntheticSection` without using any features.
Can we just use a standalone class like `ObjCSelRefsHelper` to assist this selector reference?
Or, do you actually plan to write something in the future?

https://github.com/llvm/llvm-project/pull/83878