[analyzer] Don't assert when mixing reinterpret_cast and derived-to-base casts.

This just adds a very simple check that if a DerivedToBase CastExpr is
operating on a value with known C++ object type, and that type is not the
base type specified in the AST, then the cast is invalid and we should
return UnknownVal.

In the future, perhaps we can have a checker that specifies that this is
illegal, but we still shouldn't assert even if the user turns that checker
off.

PR14872

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@175239 91177308-0d34-0410-b5e6-96231b3b80d8