[analyzer] Do not report CFError 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 NonNullParamChecker because it already checks parameter attributes.
Whenever we start analyzing a new function, we assume that all parameters
with 'nonnull' attribute are indeed non-null.

Patch by Valeriy Savchenko!

Differential Revision: https://reviews.llvm.org/D77806