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/i32.wgsl.expected.spvasm

@@ -0,0 +1,32 @@
+; SPIR-V
+; Version: 1.3
+; Generator: Google Tint Compiler; 0
+; Bound: 17
+; Schema: 0
+               OpCapability Shader
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint GLCompute %main "main"
+               OpExecutionMode %main LocalSize 1 1 1
+               OpName %main "main"
+               OpName %zero "zero"
+               OpName %init "init"
+               OpDecorate %_arr_int_uint_3 ArrayStride 4
+       %void = OpTypeVoid
+          %1 = OpTypeFunction %void
+        %int = OpTypeInt 32 1
+       %uint = OpTypeInt 32 0
+     %uint_3 = OpConstant %uint 3
+%_arr_int_uint_3 = OpTypeArray %int %uint_3
+%_ptr_Function__arr_int_uint_3 = OpTypePointer Function %_arr_int_uint_3
+         %11 = OpConstantNull %_arr_int_uint_3
+      %int_1 = OpConstant %int 1
+      %int_2 = OpConstant %int 2
+      %int_3 = OpConstant %int 3
+         %15 = OpConstantComposite %_arr_int_uint_3 %int_1 %int_2 %int_3
+       %main = OpFunction %void None %1
+          %4 = OpLabel
+       %zero = OpVariable %_ptr_Function__arr_int_uint_3 Function %11
+       %init = OpVariable %_ptr_Function__arr_int_uint_3 Function %11
+               OpStore %init %15
+               OpReturn
+               OpFunctionEnd