[swift-dev] Turns out Incremental builds were not actually incremental

[Michael Gottesman]

Given the backup in the CI, I started looking at what was going on there and I found that the following incremental bots were actually /NOT/ incremental:

1. OS X Swift incremental.
2. 14.04 Linux Swift incremental.
3. 15.10 Linux Swift incremental.
4. OSX LLDB incremental

This is in addition to the smoke test bot which was also made non-incremental (I fixed it yesterday).

These are now incremental as they should be. This should reduce the load on the CI system.

Michael