Allow import of arbitrary opaque typedefs

[jameysharp]

We can declare an opaque struct type, but for something like C's FILE *, we need to be able to pull in the type FILE without any qualifiers like struct.

Pat proposes, and this sounds good to me, that we add another Area kind or something for opaque typedefs. The declaration should indicate which header file the type comes from.

[Hodapp87]

I would second (third?) this. I'm using void* for this right now.