GLSL: move entry point handling to CanonicalizeEntryPointIO transform.

Move builtin_to_string() and builtin_type() to
the CanonicalizeEntryPointIO transform. Use the former to
rename entry point IO variables to the gl_ names, and the latter
to cast values to the correct type.

Change-Id: Iddfad574ddd660ff1bfd89a399a001b967b6b67e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/78380
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
diff --git a/test/intrinsics/gen/cos/16dc15.wgsl.expected.glsl b/test/intrinsics/gen/cos/16dc15.wgsl.expected.glsl
index 68bc55b..9f3d3d9 100644
--- a/test/intrinsics/gen/cos/16dc15.wgsl.expected.glsl
+++ b/test/intrinsics/gen/cos/16dc15.wgsl.expected.glsl
@@ -5,31 +5,18 @@
   vec3 res = cos(vec3(0.0f, 0.0f, 0.0f));
 }
 
-struct tint_symbol {
-  vec4 value;
-};
-
-vec4 vertex_main_inner() {
+vec4 vertex_main() {
   cos_16dc15();
   return vec4(0.0f, 0.0f, 0.0f, 0.0f);
 }
 
-tint_symbol vertex_main() {
-  vec4 inner_result = vertex_main_inner();
-  tint_symbol wrapper_result = tint_symbol(vec4(0.0f, 0.0f, 0.0f, 0.0f));
-  wrapper_result.value = inner_result;
-  return wrapper_result;
-}
-
-
 void main() {
-  tint_symbol outputs;
-  outputs = vertex_main();
-  gl_Position = outputs.value;
-  gl_Position.z = 2.0 * gl_Position.z - gl_Position.w;
-  gl_Position.y = -gl_Position.y;
+  vec4 inner_result = vertex_main();
+  gl_Position = inner_result;
+  gl_Position.y = -(gl_Position.y);
+  gl_Position.z = ((2.0f * gl_Position.z) - gl_Position.w);
+  return;
 }
-
 #version 310 es
 precision mediump float;
 
@@ -37,19 +24,14 @@
   vec3 res = cos(vec3(0.0f, 0.0f, 0.0f));
 }
 
-struct tint_symbol {
-  vec4 value;
-};
-
 void fragment_main() {
   cos_16dc15();
-  return;
 }
 
 void main() {
   fragment_main();
+  return;
 }
-
 #version 310 es
 precision mediump float;
 
@@ -57,17 +39,12 @@
   vec3 res = cos(vec3(0.0f, 0.0f, 0.0f));
 }
 
-struct tint_symbol {
-  vec4 value;
-};
-
-layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void compute_main() {
   cos_16dc15();
-  return;
 }
 
+layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void main() {
   compute_main();
+  return;
 }
-