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/buffer/uniform/dynamic_index/read.wgsl.expected.wgsl

@@ -12,11 +12,11 @@ struct Inner {
   j : mat3x2<f32>;
   [[align(16)]]
   k : [[stride(16)]] array<vec4<i32>, 4>;
-};
+}
 
 struct S {
   arr : array<Inner, 8>;
-};
+}
 
 [[binding(0), group(0)]] var<uniform> s : S;