[objc-arc] Create two subclasses of PtrState in preparation for moving per ptr...

[objc-arc] Create two subclasses of PtrState in preparation for moving per ptr state change behavior onto a PtrState class.

This will enable the main ObjCARCOpts dataflow to work with higher
level concepts such as "can this ptr state be modified by this ref
count" and not need to understand the nitty gritty details of how that
is determined. This makes the dataflow cleaner.

llvm-svn: 231437