Mark operator== const to avoid errors when asserts are enabled

Without this change, the build will fail like so:

llvm-project/lld/MachO/ObjC.cpp:1387:75: error: ISO C++20 considers use of overloaded operator '==' (with operand types 'ObjcCategoryMerger::PointerListInfo' and 'ObjcCategoryMerger::PointerListInfo') to be ambiguous despite there being a unique best viable function [-Werror,-Wambiguous-reversed-operator]
 1387 |       parseProtocolListInfo(classIsec, roClassLayout.baseProtocolsOffset) ==
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
 1388 |           parseProtocolListInfo(metaIsec, roClassLayout.baseProtocolsOffset) &&
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/assert.h:100:27: note: expanded from macro 'assert'
  100 |      (static_cast <bool> (expr)                                         \
      |                           ^~~~
llvm-project/lld/MachO/ObjC.cpp:391:17: note: ambiguity is between a regular call to this operator and a call with the argument order reversed
  391 |     inline bool operator==(const PointerListInfo &cmp) {
      |                 ^
llvm-project/lld/MachO/ObjC.cpp:391:17: note: mark 'operator==' as const or add a matching 'operator!=' to resolve the ambiguity
1 error generated.

Author: Sterling-Augustine

lld/MachO/ObjC.cpp

@@ -388,7 +388,7 @@ class ObjcCategoryMerger {
         : categoryPrefix(_categoryPrefix),
           pointersPerStruct(_pointersPerStruct) {}
 
-    inline bool operator==(const PointerListInfo &cmp) {
+    inline bool operator==(const PointerListInfo &cmp) const {
       return pointersPerStruct == cmp.pointersPerStruct &&
              structSize == cmp.structSize && structCount == cmp.structCount &&
              allPtrs == cmp.allPtrs;