transform/shader_io: Generate a wrapper function

This is a major reworking of this transform. The old transform code
was getting unwieldy, with part of the complication coming from the
handling of multiple return statements. By generating a wrapper
function instead, we can avoid a lot of this complexity.

The original entry point function is stripped of all shader IO
attributes (as well as `stage` and `workgroup_size`), but the body is
left unmodified. A new entry point wrapper function is introduced
which calls the original function, packing/unpacking the shader inputs
as necessary, and propagates the result to the corresponding shader
outputs.

The new code has been refactored to use a state object with the
different parts of the transform split into separate functions, which
makes it much more manageable.

Fixed: tint:1076
Bug: tint:920
Change-Id: I3490a0ea7a3509a4e198ce730e476516649d8d96
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60521
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/test/shader_io/vertex_input_locations.wgsl.expected.hlsl b/test/shader_io/vertex_input_locations.wgsl.expected.hlsl
index e604ee9..94c8220 100644
--- a/test/shader_io/vertex_input_locations.wgsl.expected.hlsl
+++ b/test/shader_io/vertex_input_locations.wgsl.expected.hlsl
@@ -8,15 +8,17 @@
   float4 value : SV_Position;
 };
 
-tint_symbol_2 main(tint_symbol_1 tint_symbol) {
-  const int loc0 = tint_symbol.loc0;
-  const uint loc1 = tint_symbol.loc1;
-  const float loc2 = tint_symbol.loc2;
-  const float4 loc3 = tint_symbol.loc3;
+float4 main_inner(int loc0, uint loc1, float loc2, float4 loc3) {
   const int i = loc0;
   const uint u = loc1;
   const float f = loc2;
   const float4 v = loc3;
-  const tint_symbol_2 tint_symbol_3 = {float4(0.0f, 0.0f, 0.0f, 0.0f)};
-  return tint_symbol_3;
+  return float4(0.0f, 0.0f, 0.0f, 0.0f);
+}
+
+tint_symbol_2 main(tint_symbol_1 tint_symbol) {
+  const float4 inner_result = main_inner(tint_symbol.loc0, tint_symbol.loc1, tint_symbol.loc2, tint_symbol.loc3);
+  tint_symbol_2 wrapper_result = (tint_symbol_2)0;
+  wrapper_result.value = inner_result;
+  return wrapper_result;
 }