Add linting for crate_type attribute values.

This ensures that the `crate_type` attribute always contains a value,
and does not contain an invalid value.

src/test/compile-fail/invalid-crate-type.rs

@@ -0,0 +1,6 @@
+// regression test for issue 11256
+#[crate_type="foo"];    //~ ERROR invalid `crate_type` value
+
+fn main() {
+    return
+}

src/test/compile-fail/no_crate_type.rs

@@ -0,0 +1,6 @@
+// regresion test for issue 11256
+#[crate_type];  //~ ERROR `crate_type` requires a value
+
+fn main() {
+    return
+}