Split `BindingPoint` out of MultiplanarExternalTexture.

This CL moves the `using` statement for `BindingPoint` out to its own
file in `src/tint/writer/binding_point.h`. Usages of
`tint::transform::BindingPoint` have been updated to
`tint::writer::BindingPoint`.

Bug: tint:1855
Change-Id: Ife7d92807e0a65b93f21158d73a60d5068fc5da9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/123000
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
diff --git a/src/tint/transform/multiplanar_external_texture.cc b/src/tint/transform/multiplanar_external_texture.cc
index 27eeeb0..800a057 100644
--- a/src/tint/transform/multiplanar_external_texture.cc
+++ b/src/tint/transform/multiplanar_external_texture.cc
@@ -121,7 +121,7 @@
             // The binding points for the newly introduced bindings must have been provided to this
             // transform. We fetch the new binding points by providing the original texture_external
             // binding points into the passed map.
-            BindingPoint bp = sem_var->BindingPoint();
+            sem::BindingPoint bp = sem_var->BindingPoint();
 
             BindingsMap::const_iterator it = new_binding_points->bindings_map.find(bp);
             if (it == new_binding_points->bindings_map.end()) {
@@ -494,6 +494,7 @@
 
 MultiplanarExternalTexture::NewBindingPoints::NewBindingPoints(BindingsMap inputBindingsMap)
     : bindings_map(std::move(inputBindingsMap)) {}
+
 MultiplanarExternalTexture::NewBindingPoints::~NewBindingPoints() = default;
 
 MultiplanarExternalTexture::MultiplanarExternalTexture() = default;