Rewrite E0019 example

Inline assembly is now the only user of E0019. What is it doing that
E0015 is not?

compiler/rustc_error_codes/src/error_codes/E0019.md

@@ -4,12 +4,14 @@ because the expression's value must be known at compile-time.
 Erroneous code example:
 
 ```compile_fail,E0019
-#![feature(box_syntax)]
+#![feature(asm)]
 
 fn main() {
-    struct MyOwned;
-
-    static STATIC11: Box<MyOwned> = box MyOwned; // error!
+    static STATIC11: i32 = {
+        let x: i32;
+        unsafe { asm!("mov rax, 2", out("rax") x) }; // error!
+        x
+    };
 }
 ```