WebIDL: Remove all typedefs in early passes

[alexcrichton]

According to @ohanar, even the WebIDl spec says we shouldn't expose them!

[ohanar]

To quote the first paragraph of the WebIDL typedef spec:

A typedef is a definition (matching Typedef) used to declare a new name for a type. This new name is not exposed by language bindings; it is purely used as a shorthand for referencing the type in the IDL.