swift234

Do not treat iOS as active on tvOS.

Change the active-platform availability logic to not consider iOS as active on
tvOS. We want all of the messiness of the fact that tvOS was branched
from iOS to be handled in clang, which "transcribes" iOS availability attributes
to their corresponding tvOS counterparts as long as there is not an
existing explicit tvOS availability attribute on the declaration.

This change exposed several places where I needed to add explicit handling of
of tvOS and where we will need to handle watchOS as well.
rdar://problem/20774229 tracks adding logic and tests to handle watchOS in these
places.

It is also unfortunately the case that llvm::triple returns true when isiOS()
is called on tvOS. This means that to test whether an llvm:triple target is
really iOS, we need to check for iOS then check explicitly that it is not
tvOS. We will eventually change llvm's behavior here so that the double
check is not needed.

Swift SVN r28013