[MLIR][Pass] Have PassRegistryEntry own pass strings

[trilorez]

Summary

This eliminates the requirement that pass-related strings outlive pass
instances, which will facilitate future work enabling dynamic passes
written in other languages.

mlir/include/mlir/Pass/PassRegistry.h

PassRegistryEntry(
       StringRef arg, StringRef description, const PassRegistryFunction &builder,
       std::function<void(function_ref<void(const detail::PassOptions &)>)>
           optHandler)
       : arg(arg), description(description), builder(builder),
         optHandler(std::move(optHandler)) {}
 
 private:
   /// The argument with which to invoke the pass via mlir-opt.
-  StringRef arg;
+  std::string arg;
 
   /// Description of the pass.
-  StringRef description;
+  std::string description;
 
   /// Function to register this entry to a pass manager pipeline.
   PassRegistryFunction builder;
 
   /// Function to invoke a handler for a pass options instance.
   std::function<void(function_ref<void(const detail::PassOptions &)>)>
       optHandler;
 };