Use the generic type lowering algorithm described in
"docs/CallingConvention.rst#physical-lowering" to map from IRGen's explosion
type to the type expected by the ABI.
Change IRGen to use the swift calling convention (swiftcc) for native swift
functions.
Use the 'swiftself' attribute on self parameters and for closures contexts.
Use the 'swifterror' parameter for swift error parameters.
Change functions in the runtime that are called as native swift functions to use
the swift calling convention.
rdar://19978563
2. Pass NSString? to thunk, not String?
3. Use proper Objective-C out parameter coding practices
4. Use correct nullability decorator in thunk for backupItemName parameter
rdar://problem/28755011
Use the portable spelling for the nullability keywords (_Nullable, _Nonnullable,
_Null_unspecified) rather than the old spelling (__nullable, __nonnullable,
__null_unspecified). NFC.
