retain/release checker:

- For autorelease pool tracking, keep information about the stack of pools
  separate from their contents. Also, keep track of the number of times an
  autorelease pool will send the "release" message to an object when the pool is
  destroyed.
- Update CFRefCount::Update to return a new state instead of a reference count
  binding. This will allow us to implement more complicated semantics with
  autorelease pools.

llvm-svn: 65384