[analyzer] Nullability: Don't warn along paths where null returned from non-null.

Change the nullability checker to not warn along paths where null is returned from
a method with a non-null return type, even when the diagnostic for this return
has been suppressed. This prevents warning from methods with non-null return types
that inline methods that themselves return nil but that suppressed the diagnostic.

Also change the PreconditionViolated state component to be called "InvariantViolated"
because it is set when a post-condition is violated, as well.

rdar://problem/25393539

llvm-svn: 264647