writer/hlsl: Zero initialize with (T) 0

For structures and arrays.
This behaves identically to the per-element zero-initialization, but can be significantly less verbose.

Change-Id: I380ef86f16c2b3f37a9de2820e707f368955b761
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56764
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

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

@@ -4,7 +4,7 @@ struct tint_array_wrapper {
 
 [numthreads(1, 1, 1)]
 void main() {
-  tint_array_wrapper v = {{0, 0, 0}};
+  tint_array_wrapper v = (tint_array_wrapper)0;
   v;
   return;
 }

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

@@ -5,7 +5,7 @@ struct S {
 
 [numthreads(1, 1, 1)]
 void main() {
-  S v = {0, 0.0f};
+  S v = (S)0;
   v;
   return;
 }