Use the portable spelling for the nullability keywords (_Nullable, _Nonnullable,
_Null_unspecified) rather than the old spelling (__nullable, __nonnullable,
__null_unspecified). NFC.
The general rule here is that something needs to be SWIFT_CC(swift)
if it's just declared in Swift code using _silgen_name, as opposed to
importing something via a header.
Of course, SWIFT_CC(swift) expands to nothing by default for now, and
I haven't made an effort yet to add the indirect-result / context
parameter ABI attributes. This is just a best-effort first pass.
I also took the opportunity to shift a few files to just implement
their shims header and to demote a few things to be private stdlib
interfaces.
This change depends on a coordinated SceneKit change which makes the
original method unavailable to avoid ambiguity. The API also requires a
data file, so no tests yet.
rdar://20384835
Swift SVN r27319
