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/textureSample/100dc0.wgsl.expected.glsl b/test/intrinsics/gen/textureSample/100dc0.wgsl.expected.glsl
index cf23155..97c7b80 100644
--- a/test/intrinsics/gen/textureSample/100dc0.wgsl.expected.glsl
+++ b/test/intrinsics/gen/textureSample/100dc0.wgsl.expected.glsl
@@ -9,10 +9,9 @@
 
 void fragment_main() {
   textureSample_100dc0();
-  return;
 }
 
 void main() {
   fragment_main();
+  return;
 }
-