libsyntax: Disallow struct literals after if, while, match, and

`for...in`.

Closes #14803.

If you used a structure literal after one of these keywords, surround it
in parentheses.

[breaking-change]

src/test/compile-fail/match-struct.rs

@@ -13,7 +13,7 @@ struct S { a: int }
 enum E { C(int) }
 
 fn main() {
-    match S { a: 1 } {
+    match (S { a: 1 }) {
         C(_) => (), //~ ERROR mismatched types: expected `S` but found `E`
         _ => ()
     }