writer/hlsl: Don't wrap arrays in structures

FXC has trouble dealing with these.
This was originally added to handle returning arrays as structures.
HLSL supports typedefs, which is a much simpiler solution, and doesn't upset FXC.

Bug: tint:848
Bug: tint:904
Change-Id: Ie841c9c454461a885a35c41476fd4d05d3f34cbf
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56774
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

test/var/initialization/function/array.wgsl.expected.hlsl

@@ -1,10 +1,6 @@
-struct tint_array_wrapper {
-  int arr[3];
-};
-
 [numthreads(1, 1, 1)]
 void main() {
-  tint_array_wrapper v = (tint_array_wrapper)0;
+  int v[3] = (int[3])0;
   v;
   return;
 }