The SyntacticMigratorPass is getting Too Big To Fail and covers
multiple migrations. There was already an early exit to not run
the pass if APIDigesterDataStorePath wasn't supplied, so SE-0110
tuple splat fix-its weren't getting run. This manifested in Migrator
tests not printing migrated contents on Linux.
New: ASTMigratorPass
Renamed: SyntacticMigratorPass -> APIDiffMigratorPass
New: TupleSplatMigratorPass
These implementations are entirely hidden and can only be invoked
by a swift::migrator function.
rdar://problem/32025974
