`convertFromSnakeCase` and `convertToSnakeCase` are not inverses of each other for keys like "pictureURL" (which roundtrips to "pictureUrl").
This does not fix the underlying issue, but improves the error message if this situation is detected.
JSONEcoder/Decoder get another underscore in their ObjC name, as the one with a single underscore conflicts with older Swifts that had _JSONEcoder/Decoder as internal rather than fileprivate.
_JSONEncoder/Decoder get another underscore in their Swift name to avoid conflicting with the ObjC name from the ones in older overlays.
Old Swift and new Swift runtimes and overlays need to coexist in the same process. This means there must not be any classes which have the same ObjC runtime name in old and new, because the ObjC runtime doesn't like name collisions.
When possible without breaking source compatibility, classes were renamed in Swift, which results in a different ObjC name.
Public classes were renamed only on the ObjC side using the @_objcRuntimeName attribute.
This is similar to the work done in pull request #19295. That only renamed @objc classes. This renames all of the others, since even pure Swift classes still get an ObjC name.
rdar://problem/46646438
The SDK directory is now confusing as the Windows target also has a SDK
overlay. In order to make this more uniform, move the SDK directory to
Darwin which covers the fact that this covers the XNU family of OSes.
The Windows directory contains the SDK overlay for the Windows target.
