Add ability to generate typedefs from dawn.json

Can be used to help with deprecation during simple struct renames.

Includes typedefs for VertexAttributeDescriptor -> VertexAttribute and
VertexBufferLayoutDescriptor -> VertexBufferLayout as specified by the
latest RenderPipelineDescriptor changes.

Bug: dawn:642
Change-Id: Iab3d74d179884499540e813b0e66859713031ccb
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/40581
Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

generator/templates/webgpu_cpp.h

@@ -63,6 +63,13 @@ namespace wgpu {
         struct {{as_cppType(type.name)}};
     {% endfor %}
 
+    {% for typeDef in by_category["typedef"] %}
+        // {{as_cppType(typeDef.name)}} is deprecated.
+        // Use {{as_cppType(typeDef.type.name)}} instead.
+        using {{as_cppType(typeDef.name)}} = {{as_cppType(typeDef.type.name)}};
+
+    {% endfor %}
+
     template<typename Derived, typename CType>
     class ObjectBase {
       public: