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/function/array/array_i32.wgsl

@@ -0,0 +1,5 @@
+@compute @workgroup_size(1)
+fn main() {
+    var zero : array<array<i32, 3>, 2>;
+    var init : array<array<i32, 3>, 2> = array<array<i32, 3>, 2>(array<i32, 3>(1, 2, 3), array<i32, 3>(4, 5, 6));
+}