Update src/tint unittests to new @stage format.

This CL updates all of the Tint unittests to the new @stage shorter
syntax. This also updates the WGSL writer to emit the new short forms
instead of using the deprecated form.

Bug: tint:1503
Change-Id: I8c49e5319a19cccb5b4b5078f3ab39c50f31a9a8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/92483
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/test/tint/builtins/gen/var/insertBits/428b0b.wgsl.expected.wgsl b/test/tint/builtins/gen/var/insertBits/428b0b.wgsl.expected.wgsl
index 2dbf357..bea28a0 100644
--- a/test/tint/builtins/gen/var/insertBits/428b0b.wgsl.expected.wgsl
+++ b/test/tint/builtins/gen/var/insertBits/428b0b.wgsl.expected.wgsl
@@ -6,18 +6,18 @@
   var res : vec3<i32> = insertBits(arg_0, arg_1, arg_2, arg_3);
 }
 
-@stage(vertex)
+@vertex
 fn vertex_main() -> @builtin(position) vec4<f32> {
   insertBits_428b0b();
   return vec4<f32>();
 }
 
-@stage(fragment)
+@fragment
 fn fragment_main() {
   insertBits_428b0b();
 }
 
-@stage(compute) @workgroup_size(1)
+@compute @workgroup_size(1)
 fn compute_main() {
   insertBits_428b0b();
 }