Convert @stage to short form

This CL converts remaining @stage instances in the dawn tree to use
the equivalent shorter variant.

Bug: tint:1503
Change-Id: I74594cd68544fbd692f77d4646991d9c27e218f8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/92484
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/dawn/native/Device.cpp

@@ -274,7 +274,7 @@ MaybeError DeviceBase::Initialize(Ref<QueueBase> defaultQueue) {
     if (IsToggleEnabled(Toggle::UsePlaceholderFragmentInVertexOnlyPipeline)) {
         // The empty fragment shader, used as a work around for vertex-only render pipeline
         constexpr char kEmptyFragmentShader[] = R"(
-                @stage(fragment) fn fs_empty_main() {}
+                @fragment fn fs_empty_main() {}
             )";
         ShaderModuleDescriptor descriptor;
         ShaderModuleWGSLDescriptor wgslDesc;