Removing descriptor set layouts from HAL IR and simplifying bindings.
* Renamed `push_constants` to `constants` (as there is no longer a
  `push_constants` API)
* Dropped `#hal.descriptor_set.layout`
* Removed ordinal from `#hal.descriptor_set.binding` (as ordinals are
  now implicit)
* Renamed `#hal.descriptor_set.binding` to `#hal.pipeline.binding`
* Removed `set` from `hal.interface.binding.subspan`
* Removed `#hal.interface.binding` and the spooky action at a distance
  `hal.interface.binding` attr now that ordinals are implicit

Progress on #18154.
diff --git a/experimental/webgpu/pipeline_layout.h b/experimental/webgpu/pipeline_layout.h
index e15b3bf..c620f9c 100644
--- a/experimental/webgpu/pipeline_layout.h
+++ b/experimental/webgpu/pipeline_layout.h
@@ -61,7 +61,7 @@
 iree_status_t iree_hal_webgpu_pipeline_layout_create(
     WGPUDevice device, iree_host_size_t set_layout_count,
     iree_hal_descriptor_set_layout_t* const* set_layouts,
-    iree_host_size_t push_constant_count,
+    iree_host_size_t constant_count,
     iree_hal_webgpu_staging_buffer_t* staging_buffer,
     iree_allocator_t host_allocator,
     iree_hal_pipeline_layout_t** out_pipeline_layout);
@@ -69,7 +69,7 @@
 WGPUPipelineLayout iree_hal_webgpu_pipeline_layout_handle(
     iree_hal_pipeline_layout_t* layout);
 
-iree_host_size_t iree_hal_webgpu_pipeline_layout_push_constant_count(
+iree_host_size_t iree_hal_webgpu_pipeline_layout_constant_count(
     iree_hal_pipeline_layout_t* layout);
 
 const iree_hal_webgpu_set_binding_info_t*