Add error reporting on nested keywords inside 'enum' definition

src/test/ui/enum/nested-enum.rs

@@ -0,0 +1,8 @@
+enum Foo {
+    enum Bar { Baz }, //~ ERROR `enum` definition cannot be nested inside `enum`
+    struct Quux { field: u8 }, //~ ERROR `struct` definition cannot be nested inside `enum`
+    union Wibble { field: u8 }, //~ ERROR `union` definition cannot be nested inside `enum`
+    Bat,
+}
+
+fn main() { }