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/private/struct.wgsl.expected.hlsl

@@ -3,7 +3,7 @@ struct S {
   float b;
 };
 
-static S v = {0, 0.0f};
+static S v = (S)0;
 
 [numthreads(1, 1, 1)]
 void main() {