Add comment for mir_opt_level=0

Author: flip1995

src/driver.rs

@@ -76,6 +76,10 @@ impl rustc_driver::Callbacks for ClippyCallbacks {
             clippy_lints::register_renamed(&mut lint_store);
         }));
 
+        // FIXME: #4825; This is required, because Clippy lints that are based on MIR have to be
+        // run on the unoptimized MIR. On the other hand this results in some false negatives. If
+        // MIR passes can be enabled / disabled separately, we should figure out, what passes to
+        // use for Clippy.
         config.opts.debugging_opts.mir_opt_level = 0;
     }
 }