swift234

[arc] Add RCStateTransition.

An RCStateTransition is (you guessed it) a transition in the state
of an RCIdentity. It abstracts operations such as:

1. The introduction of a new incremented RCID by an argument (and in
the future
an @owned return value).
2. Strong increment of an RC.
3. Strong decrement of an RC.

I will use this to move pin_removal into the ARC optimizer and will
open up the ARC optimizer to being able to match up unowned values in the
future. It additionally allows me to deduplicate code from
{TopDown,BottomUp}RefCountState into RefCountState, RCStateTransition.

Swift SVN r26608