[NFCi] Iterative Outliner + clang-format refactoring.

Prior to D69446 I had done some NFC cleanup to make landing an iterative
outliner a cleaner more straight-forward patch. Since then, it seems that has
landed but I noticed some ways it could be cleaned up. Specifically:

1) doOutline was meant to be the re-runable function, but instead
   runOnceOnModule was created that just calls doOutline.
2) In D69446 we discussed that the flag allowing the re-run of the
   outliner should be a flag to tell how many additional times to run
   the outliner again, not the total number of times. I don't think it
   makes sense to introduce a flag, but print an error if the flag is
   set to 0.

This is an NFCi, the i being that I get rid of the way that the
machine-outline-runs flag could be used to tell the outliner to not run
at all, and because I renamed the flag to '-machine-outliner-reruns'.

Differential Revision: https://reviews.llvm.org/D79070