Change RetainCountChecker to eagerly "escape" retained objects when they are

assigned to a struct.  This is fallout from inlining results, which expose
far more patterns where people stuff CF objects into structs and pass them
around (and we can reason about it).  The problem is that we don't have
a general way to detect when values have escaped, so as an intermediate step
we need to eagerly prune out such tracking.

Fixes <rdar://problem/11104566>.

llvm-svn: 153489