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/array/function_parameter.wgsl.expected.msl

@@ -1,34 +1,35 @@
 #include <metal_stdlib>
 
 using namespace metal;
-struct tint_array_wrapper {
-  float arr[4];
+
+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];
 };
 
-float f1(tint_array_wrapper a) {
-  return a.arr[3];
+float f1(tint_array<float, 4> a) {
+  return a[3];
 }
 
-struct tint_array_wrapper_1 {
-  tint_array_wrapper arr[3];
-};
-
-float f2(tint_array_wrapper_1 a) {
-  return a.arr[2].arr[3];
+float f2(tint_array<tint_array<float, 4>, 3> a) {
+  return a[2][3];
 }
 
-struct tint_array_wrapper_2 {
-  tint_array_wrapper_1 arr[2];
-};
-
-float f3(tint_array_wrapper_2 a) {
-  return a.arr[1].arr[2].arr[3];
+float f3(tint_array<tint_array<tint_array<float, 4>, 3>, 2> a) {
+  return a[1][2][3];
 }
 
 kernel void tint_symbol() {
-  tint_array_wrapper const a1 = {.arr={}};
-  tint_array_wrapper_1 const a2 = {.arr={}};
-  tint_array_wrapper_2 const a3 = {.arr={}};
+  tint_array<float, 4> const a1 = tint_array<float, 4>{};
+  tint_array<tint_array<float, 4>, 3> const a2 = tint_array<tint_array<float, 4>, 3>{};
+  tint_array<tint_array<tint_array<float, 4>, 3>, 2> const a3 = tint_array<tint_array<tint_array<float, 4>, 3>, 2>{};
   float const v1 = f1(a1);
   float const v2 = f2(a2);
   float const v3 = f3(a3);