swift234

Fix <rdar://problem/25168818> Don't import None members in NS_OPTIONS types

When importing members of an NS_OPTIONS (aka an option set), mark imported
members that have a value of 0 with an unavailable error.  This produces an
experience like this:

  x = NSRuncingOptions.none // expected-error {{'none' is unavailable: use [] to construct an empty option set}}

This is important to do, because option set members with a value of zero
do not act like members of the option set.  For example, they always fail a
"contains" check.