[analyzer] Correct modelling of OSDynamicCast: eagerly state split

Previously, OSDynamicCast was modeled as an identity.

This is not correct: the output of OSDynamicCast may be zero even if the
input was not zero (if the class is not of desired type), and thus the
modeling led to false positives.

Instead, we are doing eager state split:
in one branch, the returned value is identical to the input parameter,
and in the other branch, the returned value is zero.

This patch required a substantial refactoring of canEval infrastructure,
as now it can return different function summaries, and not just true/false.

rdar://45497400

Differential Revision: https://reviews.llvm.org/D53624

llvm-svn: 345338