swift234

Make call graph edge sets be SmallPtrSets rather than SmallVectors.

Re-apply r26072 with a fix to ensure that we get stable bottom-up
orderings in the call grpah.

Using SmallPtrSet here makes sense, and paves the way to more reasonable
code for editing the call graph by clients.

One unfortunate casualty of this is the ArrayRevView that was used by
one client.

Swift SVN r26093