writer/msl: Hoist array and struct initializers

Pull the HLSL transformation out to a standalone transform that can be
used by both HLSL and MSL.

The new E2E tests do not yet pass for MSL because they produce array
assignments, which will be addressed in the next patch.

Fixed: tint:826
Change-Id: Idc27c81ad45e3d4ab96d82663927d2fc1384618e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/52842
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

test/samples/cube.wgsl.expected.msl

@@ -32,11 +32,13 @@ vertex tint_symbol_2 vtx_main(tint_symbol_1 tint_symbol [[stage_in]], constant U
   VertexOutput output = {};
   output.Position = (uniforms.modelViewProjectionMatrix * input.cur_position);
   output.vtxFragColor = input.color;
-  return {output.vtxFragColor, output.Position};
+  tint_symbol_2 const tint_symbol_6 = {output.vtxFragColor, output.Position};
+  return tint_symbol_6;
 }
 
 fragment tint_symbol_5 frag_main(tint_symbol_4 tint_symbol_3 [[stage_in]]) {
   float4 const fragColor = tint_symbol_3.fragColor;
-  return {fragColor};
+  tint_symbol_5 const tint_symbol_7 = {fragColor};
+  return tint_symbol_7;
 }
 

test/samples/simple.wgsl.expected.msl

@@ -11,6 +11,7 @@ void bar() {
 fragment tint_symbol_1 tint_symbol() {
   float2 a = float2();
   bar();
-  return {float4(0.400000006f, 0.400000006f, 0.800000012f, 1.0f)};
+  tint_symbol_1 const tint_symbol_2 = {float4(0.400000006f, 0.400000006f, 0.800000012f, 1.0f)};
+  return tint_symbol_2;
 }