Objective-C's 'id' is a very odd type, because it effectively acts as
both a "top" and a "bottom" for the lattice of Objective-C types,
where "top" is a type everything is a subtype of an "bottom" is a type
everything is a supertype of. We almost surely don't want the "bottom"
behavior in Swift, but we also don't have a suitable notion of "top"
yet. protocol<> is closest, but that's too general because it also
allows value types. NSObject is the closest thing we have, for now, so
we use it.
Swift SVN r3260
There is no protection whatsoever if the Clang-to-Swift type
conversion produces something that Swift doesn't lower in an
ABI-compatible way. That will be dealt with later.
Swift SVN r3249
This importer handles all of the Clang structural types, e.g., builtin
types (int, float, void), function types, block pointer types, and
C pointer types. It does not yet handle nominal types such as enums,
structs, or Objective-C classes, and there are some questions about
(e.g.) array types.
Swift SVN r3212
