wgsl: Deprecate '-> void' on functions

Was removed with:
https://github.com/gpuweb/gpuweb/pull/1460

Bug: tint:677
Change-Id: If08cb450189c6158561051ef6e8f2439c60bc010
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/47140
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

test/compute_boids.wgsl

@@ -60,7 +60,7 @@ fn frag_main() -> [[location(0)]] vec4<f32> {
 // https://github.com/austinEng/Project6-Vulkan-Flocking/blob/master/data/shaders/computeparticles/particle.comp
 [[stage(compute)]]
 fn comp_main(
-  [[builtin(global_invocation_id)]] gl_GlobalInvocationID : vec3<u32>) -> void {
+  [[builtin(global_invocation_id)]] gl_GlobalInvocationID : vec3<u32>) {
   var index : u32 = gl_GlobalInvocationID.x;
   if (index >= 5u) {
     return;

test/function.wgsl

@@ -18,5 +18,5 @@ fn main() -> f32 {
 
 [[stage(compute)]]
 [[workgroup_size(2)]]
-fn ep() -> void {
+fn ep() {
 }

test/simple.wgsl

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-fn bar() -> void {
+fn bar() {
 }
 
 [[stage(fragment)]]