[analyzer] Trust _Nonnull annotations for system framework

[analyzer] Trust _Nonnull annotations for system framework

Changes the analyzer to believe that methods annotated with _Nonnull
from system frameworks indeed return non null objects.
Local methods with such annotation are still distrusted.
rdar://24291919

Differential Revision: https://reviews.llvm.org/D44341

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@328282 91177308-0d34-0410-b5e6-96231b3b80d8