Fix clippy lints

compiler/rustc_llvm/src/lib.rs

@@ -17,6 +17,10 @@ impl RustString {
     pub fn len(&self) -> usize {
         self.bytes.borrow().len()
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.bytes.borrow().is_empty()
+    }
 }
 
 /// Appending to a Rust string -- used by RawRustStringOstream.