Cleanup comments and braces. No functionality changed, just trying to
record what is *actually* going on here as the comments existing in the
code are confusing at best, and in places outright misleading.

The API is even more misleading. Yay.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193577 91177308-0d34-0410-b5e6-96231b3b80d8