[analyzer] Nullability: Treat nil _Nonnull ivar as invariant violation.

Treat a _Nonnull ivar that is nil as an invariant violation in a similar
fashion to how a nil _Nonnull parameter is treated as a precondition violation.

This avoids warning on defensive returns of nil on defensive internal
checks, such as the following common idiom:

@class InternalImplementation
@interface PublicClass {
  InternalImplementation * _Nonnull _internal;
}
-(id _Nonnull)foo;
@end

@implementation PublicClass
-(id _Nonnull)foo {
  if (!_internal)
    return nil; // no-warning

  return [_internal foo];
}
@end

rdar://problem/24485171

llvm-svn: 266157