rustc: Track stability of trait implementations

Previously an implementation of a stable trait allows implementations of
unstable methods. This updates the stability pass to ensure that all items of an
impl block of a trait are indeed stable on the trait itself.

src/test/compile-fail/lint-stability.rs

@@ -133,6 +133,11 @@ mod cross_crate {
     impl UnstableTrait for S { } //~ WARNING use of unstable library feature
 
     trait LocalTrait : UnstableTrait { } //~ WARNING use of unstable library feature
+
+    impl Trait for S {
+        fn trait_stable(&self) {}
+        fn trait_unstable(&self) {} //~ WARNING use of unstable library feature
+    }
 }
 
 mod inheritance {