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/shader_io/interpolate_input_struct.wgsl b/test/tint/shader_io/interpolate_input_struct.wgsl
index 46546a4..93cfec8 100644
--- a/test/tint/shader_io/interpolate_input_struct.wgsl
+++ b/test/tint/shader_io/interpolate_input_struct.wgsl
@@ -9,6 +9,6 @@
   @location(7) @interpolate(linear, sample) linear_sample : f32,
 };
 
-@stage(fragment)
+@fragment
 fn main(in : In) {
 }