[analyzer] Migrate NSErrorChecker and DereferenceChecker to CheckerV2.

They cooperate in that NSErrorChecker listens for ImplicitNullDerefEvent events that
DereferenceChecker can dispatch.
ImplicitNullDerefEvent is when we dereferenced a location that may be null.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126659 91177308-0d34-0410-b5e6-96231b3b80d8
22 files changed