[lld] Use llvm::less_first (NFC) (#136397)

Author: kazutakahirata

lld/MachO/MapFile.cpp

@@ -116,9 +116,7 @@ static MapInfo gatherMapInfo() {
   // cstrings are not stored in sorted order in their OutputSections, so we sort
   // them here.
   for (auto &liveCStrings : info.liveCStringsForSection)
-    parallelSort(liveCStrings.second, [](const auto &p1, const auto &p2) {
-      return p1.first < p2.first;
-    });
+    parallelSort(liveCStrings.second, llvm::less_first());
   return info;
 }