Add let_underscore_lock lint.

Similar to `let_underscore_drop`, this lint checks for statements similar
to `let _ = foo`, where `foo` is a lock guard. These types of let
statements are especially problematic because the lock gets released
immediately, instead of at the end of the scope. This behavior is almost
always the wrong thing.

compiler/rustc_lint/src/lib.rs

@@ -317,7 +317,7 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         REDUNDANT_SEMICOLONS
     );
 
-    add_lint_group!("let_underscore", LET_UNDERSCORE_DROP);
+    add_lint_group!("let_underscore", LET_UNDERSCORE_DROP, LET_UNDERSCORE_LOCK);
 
     add_lint_group!(
         "rust_2018_idioms",