This involved adding a new substitution called %api_diff_data_dir that when
building against a host toolchain, looks in the host toolchain (next to swiftc)
rather than in the resource dir. The reason why I need to do this is this allows
me to perform a stdlib stage2 build without needing to build swift itself.
The only interesting changes here are that I had to add %api_diff_data_dir to
a bunch of normal/expected tests and also add %api_diff_data_dir's length to the
offsets in rdar31892850.swift.
To support migration from both Swift 3 and 4, this patch
teaches the driver to pick up the right set of migration scripts
according to the given Swift version. We also pushed some placeholder script
files for migration from Swift 4. This patch also brings the migrator
up-to-date by avoiding migration if the Swift version is already 4.2.
