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/bug/fxc/indexed_assign_to_array_in_struct/1206.wgsl.expected.wgsl

@@ -1,17 +1,17 @@
 struct Simulation {
   i : u32;
-};
+}
 
 struct Particle {
   position : array<vec3<f32>, 8>;
   lifetime : f32;
   color : vec4<f32>;
   velocity : vec3<f32>;
-};
+}
 
 struct Particles {
   p : array<Particle>;
-};
+}
 
 [[group(1), binding(3)]] var<storage, read> particles : Particles;