Add code to PlatformDarwin and HostInfoMacOSX so they return the

correct OS type when running on an apple tv or apple watch.
Also, in TargetList::CreateTargetInternal, check that a platform
is returned by GetPlatformForArchitecture fallback instead of 
adding it to the vector of platforms unconditionally; we can end up
crashing when we call a member function on it later.

<rdar://problem/23601982>, <rdar://problem/21292886>

llvm-svn: 253763