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.msl

@@ -0,0 +1,32 @@
+#include <metal_stdlib>
+
+using namespace metal;
+
+template<typename T, size_t N>
+struct tint_array {
+    const constant T& operator[](size_t i) const constant { return elements[i]; }
+    device T& operator[](size_t i) device { return elements[i]; }
+    const device T& operator[](size_t i) const device { return elements[i]; }
+    thread T& operator[](size_t i) thread { return elements[i]; }
+    const thread T& operator[](size_t i) const thread { return elements[i]; }
+    threadgroup T& operator[](size_t i) threadgroup { return elements[i]; }
+    const threadgroup T& operator[](size_t i) const threadgroup { return elements[i]; }
+    T elements[N];
+};
+
+void tint_symbol_inner(uint local_invocation_index, threadgroup tint_array<tint_array<int, 3>, 2>* const tint_symbol_1) {
+  for(uint idx = local_invocation_index; (idx < 6u); idx = (idx + 1u)) {
+    uint const i = (idx / 3u);
+    uint const i_1 = (idx % 3u);
+    (*(tint_symbol_1))[i][i_1] = 0;
+  }
+  threadgroup_barrier(mem_flags::mem_threadgroup);
+  tint_array<tint_array<int, 3>, 2> v = *(tint_symbol_1);
+}
+
+kernel void tint_symbol(uint local_invocation_index [[thread_index_in_threadgroup]]) {
+  threadgroup tint_array<tint_array<int, 3>, 2> tint_symbol_2;
+  tint_symbol_inner(local_invocation_index, &(tint_symbol_2));
+  return;
+}
+