swift_retain and objc_retain both handle nil already, so we can bypass the generic Optional value witnesses when instantiating Optional on a class type, which happens kind of a lot.
Swift SVN r23613
If, after layout, we recognize that a generic struct or enum instance is POD, we can replace the generic value witnesses in the image with POD ones.
Swift SVN r23609
Set a bit for types that are non-bitwise-takable, and calculate it as part of runtime struct and enum layout. Include 'bitwise takable' as part of the runtime 'is inline' calculation to be consistent with the compile-time policy change in r17008.
Swift SVN r17036
