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>

test/expressions/zero_init/array/bool.wgsl.expected.glsl

@@ -5,11 +5,6 @@ layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {
   return;
 }
-
-void main() {
-  unused_entry_point();
-}
-
 void f() {
   bool v[4] = bool[4](false, false, false, false);
 }

test/expressions/zero_init/array/f32.wgsl.expected.glsl

@@ -5,11 +5,6 @@ layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {
   return;
 }
-
-void main() {
-  unused_entry_point();
-}
-
 void f() {
   float v[4] = float[4](0.0f, 0.0f, 0.0f, 0.0f);
 }

test/expressions/zero_init/array/i32.wgsl.expected.glsl

@@ -5,11 +5,6 @@ layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {
   return;
 }
-
-void main() {
-  unused_entry_point();
-}
-
 void f() {
   int v[4] = int[4](0, 0, 0, 0);
 }

test/expressions/zero_init/array/struct.wgsl.expected.glsl

@@ -5,11 +5,6 @@ layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {
   return;
 }
-
-void main() {
-  unused_entry_point();
-}
-
 struct S {
   int i;
   uint u;

test/expressions/zero_init/array/u32.wgsl.expected.glsl

@@ -5,11 +5,6 @@ layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
 void unused_entry_point() {
   return;
 }
-
-void main() {
-  unused_entry_point();
-}
-
 void f() {
   uint v[4] = uint[4](0u, 0u, 0u, 0u);
 }