[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_core/src/main/native/cpp/WSProvider_SimDevice.cpp

@@ -54,7 +54,7 @@ void HALSimWSProviderSimDevice::CancelCallbacks() {
   m_simValueCreatedCbKey = 0;
 
   for (auto& kv : m_simValueChangedCbKeys) {
-    HALSIM_CancelSimValueChangedCallback(kv.getValue());
+    HALSIM_CancelSimValueChangedCallback(kv.second);
   }
 
   m_simValueChangedCbKeys.clear();

simulation/halsim_ws_core/src/main/native/include/WSProviderContainer.h

@@ -40,7 +40,7 @@ class ProviderContainer {
   void ForEach(IterFn fn) {
     std::shared_lock lock(m_mutex);
     for (auto& kv : m_providers) {
-      fn(kv.getValue());
+      fn(kv.second);
     }
   }