RetainCountChecker models a group of calls that are interpreted as
"identity" functions. bugreporter::Tracker couldn't see through
such calls and pick the right argument for tracking.
This commit introduces a new tag that helps to keep this information
from the actual analysis and reuse it later for tracking purposes.
So, in short, when we see a call foo(x) that we model as
"identity", we now add a tag that is saying that the result of
foo(x) is essentially x. When the tracker, later on, tries to
track value from foo(x), we point out that it should track x.