rustdoc: handle generics better when matching notable traits

This commit makes the `clean::Type::is_same` non-commutative, so
that a generic `impl` matches a concrete return, but a generic return
does not match a concrete `impl`. It makes slice and vector Write
for `u8` not match on every generic return value.

tests/rustdoc/notable-trait/notable-trait-generics.rs

@@ -0,0 +1,35 @@
+#![feature(doc_notable_trait)]
+
+// Notable traits SHOULD be shown when the `impl` has a generic type and the
+// return type has a concrete type.
+pub mod generic_return {
+    pub struct Wrapper<T>(T);
+
+    #[doc(notable_trait)]
+    pub trait NotableTrait {}
+
+    impl NotableTrait for Wrapper<u8> {}
+
+    // @has notable_trait_generics/generic_return/fn.returning.html
+    // @!has - '//a[@class="tooltip"]/@data-notable-ty' 'Wrapper<T>'
+    pub fn returning<T>() -> Wrapper<T> {
+        loop {}
+    }
+}
+
+// Notable traits SHOULD NOT be shown when the `impl` has a concrete type and
+// the return type has a generic type.
+pub mod generic_impl {
+    pub struct Wrapper<T>(T);
+
+    #[doc(notable_trait)]
+    pub trait NotableTrait {}
+
+    impl<T> NotableTrait for Wrapper<T> {}
+
+    // @has notable_trait_generics/generic_impl/fn.returning.html
+    // @has - '//a[@class="tooltip"]/@data-notable-ty' 'Wrapper<u8>'
+    pub fn returning() -> Wrapper<u8> {
+        loop {}
+    }
+}