[lldb] Use async output & error stream for EvaluateExpression

Similar to llvm#126821, in support of llvm#126630.

(cherry picked from commit ed32d85)

Author: JDevlieghere

lldb/source/Commands/CommandObjectExpression.cpp

@@ -526,19 +526,17 @@ bool CommandObjectExpression::EvaluateExpression(llvm::StringRef expr,
 void CommandObjectExpression::IOHandlerInputComplete(IOHandler &io_handler,
                                                      std::string &line) {
   io_handler.SetIsDone(true);
-  StreamFileSP output_sp = io_handler.GetOutputStreamFileSP();
-  StreamFileSP error_sp = io_handler.GetErrorStreamFileSP();
+  StreamSP output_stream =
+      GetCommandInterpreter().GetDebugger().GetAsyncOutputStream();
+  StreamSP error_stream =
+      GetCommandInterpreter().GetDebugger().GetAsyncErrorStream();
 
   CommandReturnObject return_obj(
       GetCommandInterpreter().GetDebugger().GetUseColor());
-  EvaluateExpression(line.c_str(), *output_sp, *error_sp, return_obj);
+  EvaluateExpression(line.c_str(), *output_stream, *error_stream, return_obj);
 
-  if (output_sp)
-    output_sp->Flush();
-  if (error_sp) {
-    *error_sp << return_obj.GetErrorString();
-    error_sp->Flush();
-  }
+  output_stream->Flush();
+  *error_stream << return_obj.GetErrorString();
 }
 
 bool CommandObjectExpression::IOHandlerIsInputComplete(IOHandler &io_handler,