tools: Return position from generated vertex shaders

This is required to generate valid MSL code, and will soon be
validated by Tint too.

Change-Id: I4c5f5c4ecb1c91131c934de1132217d9f6be1f8e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53420
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/test/intrinsics/gen/textureStore/5a2f8f.wgsl.expected.wgsl b/test/intrinsics/gen/textureStore/5a2f8f.wgsl.expected.wgsl
index 3053085..adeefea 100644
--- a/test/intrinsics/gen/textureStore/5a2f8f.wgsl.expected.wgsl
+++ b/test/intrinsics/gen/textureStore/5a2f8f.wgsl.expected.wgsl
@@ -5,8 +5,9 @@
 }
 
 [[stage(vertex)]]
-fn vertex_main() {
+fn vertex_main() -> [[builtin(position)]] vec4<f32> {
   textureStore_5a2f8f();
+  return vec4<f32>();
 }
 
 [[stage(fragment)]]