Add a test for associated existential type

Author: topecongiro

tests/source/existential_type.rs

@@ -9,3 +9,7 @@ where
     : Fn(T) -> T;
 
 pub existential type Adderrr<T>: Fn(  T  ) -> T;
+
+impl Foo for Bar {
+existential type E  : Trait;
+}

tests/target/existential_type.rs

@@ -9,3 +9,7 @@ where
 : Fn(T) -> T;
 
 pub existential type Adderrr<T>: Fn(T) -> T;
+
+impl Foo for Bar {
+    existential type E: Trait;
+}