[analyzer] Do not report NSError null dereference for _Nonnull params.

We want to trust user type annotations and stop assuming pointers declared
as _Nonnull still can be null. This functionality is implemented as part
of NullabilityChecker as it already tracks non-null types.

Patch by Valeriy Savchenko!

Differential Revision: https://reviews.llvm.org/D77722