[lldb] Log the actual expression result in UserExpression::Evaluate

This used to be a LLDB_LOGF call that used the printf %s syntax.
0ab109d changed it to LLDB_LOG but didn't
update this format string to use formatv's syntax so this just printed '%s'.

Author: Teemperor

lldb/source/Expression/UserExpression.cpp

@@ -362,7 +362,7 @@ UserExpression::Evaluate(ExecutionContext &exe_ctx,
 
           LLDB_LOG(log,
                    "== [UserExpression::Evaluate] Execution completed "
-                   "normally with result %s ==",
+                   "normally with result {0} ==",
                    result_valobj_sp->GetValueAsCString());
         } else {
           LLDB_LOG(log, "== [UserExpression::Evaluate] Execution completed "