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>

test/tint/shader_io/shared_struct_helper_function.wgsl

@@ -7,12 +7,12 @@ fn foo(x : f32) -> VertexOutput {
   return VertexOutput(vec4<f32>(x, x, x, 1.0), 42);
 }
 
-@stage(vertex)
+@vertex
 fn vert_main1() -> VertexOutput {
   return foo(0.5);
 }
 
-@stage(vertex)
+@vertex
 fn vert_main2() -> VertexOutput {
   return foo(0.25);
 }