swift234

Merge tvOS and watchOS Support

- Add frontend and standard library build support for tvOS.
- Add frontend support for watchOS.

watchOS standard library builds are still disabled during SDK bring-up.

To build for TVOS, specify --tvos to build-script.
To build for watchOS, specify --watchos to build-script (not yet supported).

This patch does not include turning on full tests for TVOS or watchOS, and
will be included in a follow-up patch.

Swift SVN r26278