[LSP] Use std::string for storing the Reply method (llvm#92464)

This was using a StringRef, which is very unsafe because the method name
might just get disposed due to the async nature of the response.
This was causing weird characters being printed in the output logs.

Author: walter-erquinigo

mlir/lib/Tools/lsp-server-support/Transport.cpp

@@ -41,7 +41,7 @@ class Reply {
   void operator()(llvm::Expected<llvm::json::Value> reply);
 
 private:
-  StringRef method;
+  std::string method;
   std::atomic<bool> replied = {false};
   llvm::json::Value id;
   JSONTransport *transport;