[wpiutil] Replace LLVM StringMap impl with std::map

As string_view operations on std::map<std::string> won't be integrated
until C++26, placeholder implementations are used which are less efficient
in a couple of situations (e.g. insert with hint).

wpilibc/src/main/native/cpp/Tracer.cpp

@@ -49,8 +49,8 @@ void Tracer::PrintEpochs(wpi::raw_ostream& os) {
     m_lastEpochsPrintTime = now;
     for (const auto& epoch : m_epochs) {
       os << fmt::format(
-          "\t{}: {:.6f}s\n", epoch.getKey(),
-          duration_cast<microseconds>(epoch.getValue()).count() / 1.0e6);
+          "\t{}: {:.6f}s\n", epoch.first,
+          duration_cast<microseconds>(epoch.second).count() / 1.0e6);
     }
   }
 }