Xcode: Fix target ordering operator to meet std::sort requirements

`std::sort` requires strict-weak ordering where `a == b` is false.

Backport commit 80d0770250 (Xcode: Fix target ordering operator to meet
std::sort requirements, 2024-10-10, v4.0.0-rc1~660^2~1) to CMake 3.31.

Source/cmGlobalXCodeGenerator.cxx

@@ -1365,6 +1365,9 @@ struct cmCompareTargets
   {
     std::string const& a = l->GetTarget()->GetName();
     std::string const& b = r->GetTarget()->GetName();
+    if (a == b) {
+      return false;
+    }
     if (a == "ALL_BUILD"_s) {
       return true;
     }