This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
If a file is run through the migration workflow a second or third time,
but fails to compile in its current state in Swift 4, it might be possible
for the previous remap file to get picked up, resulting in the old changes
getting applied twice or at the wrong offsets. At the start of the migration,
proactively remove the remap file to prevent this.
rdar://problem/32545844