[analyzer] CastValueChecker: Model casts

Summary:
It models the LLVM casts:
- `cast<>`
- `dyn_cast<>`
- `cast_or_null<>`
- `dyn_cast_or_null<>`

It has a very basic support without checking the `classof()` function.

Reviewed By: NoQ

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64374

llvm-svn: 365582