Support the zero initializer syntax.

This Cl updates the system to allow zero initializers. This allows:

```
var a : vec3<f32> = vec3<f32>();
```

Bug: tint:34
Change-Id: I84d6b431914c4ddf112ed375fae028d912f4a080
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23660
Reviewed-by: David Neto <dneto@google.com>

test/simple.wgsl

@@ -15,6 +15,7 @@
 [[location 0]] var<out> gl_FragColor : vec4<f32>;
 
 fn main() -> void {
+    var a : vec2<f32> = vec2<f32>();
     gl_FragColor = vec4<f32>(0.4, 0.4, 0.8, 1.0);
     return;
 }