tint/writer/msl: Generate an array<T,N> helper

And remove the WrapArraysInStructs transform.

Wrapping arrays in structures becomes troublesome for `const` arrays, as
currently WGSL does not allow `const` structures.

MSL 2.0+ has a builtin array<> helper, but we're targetting MSL 1.2, so
we have to emit our own. Fortunately, it can be done with a few lines of
templated code.

This produces significantly cleaner output.

Change-Id: Ifc92ef21e09befa252a07c856c4b5afdc51cc2e4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94540
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: David Neto <dneto@google.com>

test/tint/var/initialization/workgroup/array/array_i32.wgsl.expected.glsl

@@ -0,0 +1,20 @@
+#version 310 es
+
+shared int zero[2][3];
+void tint_symbol(uint local_invocation_index) {
+  {
+    for(uint idx = local_invocation_index; (idx < 6u); idx = (idx + 1u)) {
+      uint i = (idx / 3u);
+      uint i_1 = (idx % 3u);
+      zero[i][i_1] = 0;
+    }
+  }
+  barrier();
+  int v[2][3] = zero;
+}
+
+layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
+void main() {
+  tint_symbol(gl_LocalInvocationIndex);
+  return;
+}