Add test for assoc fn suggestion on enum variant

Author: estebank

tests/ui/enum/assoc-fn-call-on-variant.rs

@@ -0,0 +1,15 @@
+#[derive(Default)]
+enum E {
+    A {},
+    B {},
+    #[default]
+    C,
+}
+
+impl E {
+    fn f() {}
+}
+
+fn main() {
+    E::A::f(); //~ ERROR failed to resolve: `A` is a variant, not a module
+}

tests/ui/enum/assoc-fn-call-on-variant.stderr

@@ -0,0 +1,14 @@
+error[E0433]: failed to resolve: `A` is a variant, not a module
+  --> $DIR/assoc-fn-call-on-variant.rs:14:8
+   |
+LL |     E::A::f();
+   |        ^ `A` is a variant, not a module
+   |
+help: there is an enum variant `E::A`; try using the variant's enum
+   |
+LL |     E();
+   |     ~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0433`.