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

[Vincent Lee via llvm-commits]

================
@@ -852,33 +836,62 @@ void ObjCStubsSection::initialize() {
   }
 }
 
+ConcatInputSection *ObjCSelRefsSection::makeSelRef(StringRef methName) {
----------------
thevinster wrote:

nit: variable naming should be `methname` to be consistent with other places. 

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