Add TAIT struct test

Author: spastorino

src/test/ui/type-alias-impl-trait/type-alias-impl-trait-struct.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+type Foo = Vec<impl Send>;
+
+fn make_foo() -> Foo {
+    vec![true, false]
+}
+
+fn main() {}