Add auto trait leakage TAIT test

Author: spastorino

src/test/ui/type-alias-impl-trait/auto-trait-leakage.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+mod m {
+    type Foo = impl std::fmt::Debug;
+
+    pub fn foo() -> Foo {
+        22_u32
+    }
+}
+
+fn is_send<T: Send>(_: T) {}
+
+fn main() {
+    is_send(m::foo());
+}