[analyzer] Model base to derived casts more precisely.

Dynamic casts are handled relatively well by the static analyzer.
BaseToDerived casts however are treated conservatively. This can cause some
false positives with the NewDeleteLeaks checker.

This patch alters the behavior of BaseToDerived casts. In case a dynamic cast
would succeed use the same semantics. Otherwise fall back to the conservative
approach.

Differential Revision: https://reviews.llvm.org/D23014

llvm-svn: 277989