[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).

glass/src/lib/native/cpp/DataSource.cpp

@@ -112,5 +112,5 @@ DataSource* DataSource::Find(std::string_view id) {
   if (it == gContext->sources.end()) {
     return nullptr;
   }
-  return it->getValue();
+  return it->second;
 }