swift234

[Clang Importer] Initial support for swift_newtype(struct)

This introduces support for swift_newtype(struct) attribute (also
known as swift_wrapper). The Clang importer will create a brand new
struct corresponding to the annotated typedefs, which has a backing
raw value. Globals of that type are imported as static members on the
struct.

Additionally, this interacts seamlessly with prior import-as-member
work, meaning that the newly created type can be imported onto. Tests
included.