wgsl: Do not require a semicolon after a struct

Don't generate them either, which generated a lot of test churn.

Fixed: tint:1380
Change-Id: I0a7cfdd2ef0ffe8e7fda111fbc57997b36b949e0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/77165
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/expressions/zero_init/array/struct.wgsl.expected.wgsl

@@ -3,7 +3,7 @@ struct S {
   u : u32;
   f : f32;
   b : bool;
-};
+}
 
 fn f() {
   var v = array<S, 4>();