[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).

simulation/halsim_ws_server/src/main/native/cpp/HALSimWeb.cpp

@@ -123,8 +123,8 @@ void HALSimWeb::Start() {
   // Print any filters we are using
   if (m_useMsgFiltering) {
     wpi::print("WS Message Filters:");
-    for (auto filter : m_msgFilters.keys()) {
-      wpi::print("* \"{}\"\n", filter);
+    for (auto&& filter : m_msgFilters) {
+      wpi::print("* \"{}\"\n", filter.first);
     }
   } else {
     wpi::print("No WS Message Filters specified");