swift234

[Clang Importer] Refactor the handling of type import contexts.

The importer has always imported types differently in different
places, e.g., function parameter types have special rules for C++
references. Formalize this notion a bit, and make it work properly
when dealing with typedefs where the underlying type itself might be
imported differently in different contexts. 

As part of this, limit the import of C's 'void' type to the result of
a function type.


Swift SVN r5055