Enable deny-warnings feature everywhere in CI

rustc_tools_util/Cargo.toml

@@ -9,4 +9,8 @@ license = "MIT OR Apache-2.0"
 keywords = ["rustc", "tool", "git", "version", "hash"]
 categories = ["development-tools"]
 edition = "2018"
+
 [dependencies]
+
+[features]
+deny-warnings = []

rustc_tools_util/src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(feature = "deny-warnings", deny(warnings))]
+
 use std::env;
 
 #[macro_export]