Fix an ICE when trying to resolve a struct variant

Unlike a tuple variant constructor which can be called as a function, a
struct variant constructor is not a function, so cannot be called.

If the user tries to assign the constructor to a variable, an ICE
occurs, because there is no way to use it later. So we should stop the
constructor from being used like that.

A similar mechanism already exists for a normal struct, as it prohibits
a struct from being resolved. This commit does the same for a struct
variant.

This commit also includes some changes to the existing tests.

Fixes #19452.

src/test/compile-fail/issue-18252.rs

@@ -13,5 +13,5 @@ enum Foo {
 }
 
 fn main() {
-    let f = Foo::Variant(42u); //~ ERROR expected function, found `Foo`
+    let f = Foo::Variant(42u); //~ ERROR uses it like a function
 }

src/test/compile-fail/issue-19452.rs

@@ -0,0 +1,17 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+enum Homura {
+    Madoka { age: u32 }
+}
+
+fn main() {
+    let homura = Homura::Madoka; //~ ERROR uses it like a function
+}