writer/msl: Wrap each array type in a struct

This allows them to be used in various places that WGSL allows, such
as function return types and parameters, and as the type of the RHS of
an assignment.

Fixed: tint:814
Fixed: tint:820
Change-Id: Idb6a901b9a34e96bb9733cc158191e7b3bafaa0e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/52844
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

test/array/assign_to_function_var.wgsl

@@ -0,0 +1,53 @@
+type ArrayType = [[stride(16)]] array<i32, 4>;
+
+[[block]]
+struct S {
+  arr : ArrayType;
+};
+
+var<private> src_private : ArrayType;
+var<workgroup> src_workgroup : ArrayType;
+[[group(0), binding(0)]] var<uniform> src_uniform : S;
+[[group(0), binding(1)]] var<storage> src_storage : [[access(read_write)]] S;
+
+fn ret_arr() -> ArrayType {
+  return ArrayType();
+}
+
+fn ret_struct_arr() -> S {
+  return S();
+}
+
+fn foo(src_param : ArrayType) {
+  var src_function : ArrayType;
+
+  var dst : ArrayType;
+
+  // Assign from type constructor.
+  dst = ArrayType(1, 2, 3, 3);
+
+  // Assign from parameter.
+  dst = src_param;
+
+  // Assign from function call.
+  dst = ret_arr();
+
+  // Assign from constant.
+  let src_let : ArrayType = ArrayType();
+  dst = src_let;
+
+  // Assign from var, various storage classes.
+  dst = src_function;
+  dst = src_private;
+  dst = src_workgroup;
+
+  // Assign from struct.arr, various storage classes.
+  dst = ret_struct_arr().arr;
+  dst = src_uniform.arr;
+  dst = src_storage.arr;
+
+  // Nested assignment.
+  var dst_nested : array<array<array<i32, 2>, 3>, 4>;
+  var src_nested : array<array<array<i32, 2>, 3>, 4>;
+  dst_nested = src_nested;
+}