- 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
Without this, clients that don't use a CompilerInstance (like LLDB) won't
have target configuration options available.
Also, move minimum OS checking into the driver. This makes sure the check
happens early (and only once), and in general fits the philosophy of
allowing the frontend to use configurations that might be banned for users.
<rdar://problem/17688913>
Swift SVN r20701
