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/ptr_ref/access/matrix.spvasm.expected.wgsl b/test/tint/ptr_ref/access/matrix.spvasm.expected.wgsl
index c5e748e..b913709 100644
--- a/test/tint/ptr_ref/access/matrix.spvasm.expected.wgsl
+++ b/test/tint/ptr_ref/access/matrix.spvasm.expected.wgsl
@@ -5,7 +5,7 @@
   return;
 }
 
-@stage(compute) @workgroup_size(1i, 1i, 1i)
+@compute @workgroup_size(1i, 1i, 1i)
 fn main() {
   main_1();
 }