Rename ignored_generic_bounds -> type_alias_bounds

First of all, the lint is specific for type aliases.  Second, it turns out the
bounds are not entirely ignored but actually used when accessing associated
types.  So change the wording of the lint, and adapt its name to reality.

The lint has never been on stable or beta, so renaming is safe.

src/librustc_lint/lib.rs

@@ -109,7 +109,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                        UnusedImportBraces,
                        AnonymousParameters,
                        UnusedDocComment,
-                       IgnoredGenericBounds,
+                       TypeAliasBounds,
                        );
 
     add_early_builtin_with_new!(sess,