Use diagnostic items instead of hard coded paths for let_underscore_lock

Using diagnostic items avoids having to update the paths if the guard
types ever get moved around for some reason. Additionally, it also greatly
simplifies the `is_sync_lock` check.

compiler/rustc_span/src/symbol.rs

@@ -213,6 +213,7 @@ symbols! {
         LinkedList,
         LintPass,
         Mutex,
+        MutexGuard,
         N,
         None,
         Ok,
@@ -250,6 +251,8 @@ symbols! {
         Right,
         RustcDecodable,
         RustcEncodable,
+        RwLockReadGuard,
+        RwLockWriteGuard,
         Send,
         SeqCst,
         SliceIndex,