llvm::Optional lives in "llvm/ADT/Optional.h". Like Clang, we can get
Optional in the 'swift' namespace by including "swift/Basic/LLVM.h".
We're now fully switched over to llvm::Optional!
Swift SVN r22477
A successor map is a somewhat targetted data structure which
is designed to give you the value for the lowest key that is
larger than the lookup key.
This is useful when applying a transformation that isn't
entirely one-to-one but where we nonetheless want to preserve
the original order as much as possible, e.g. when IR-genning
SILFunctions.
Swift SVN r14199
