swift234

Use preferred pattern for operator| on the enum used in a swift::OptionSet.

...and add a static_assert to OptionSet to enforce it. There are two holes
allowed: if it's not an enum class, the default operator| is acceptable, and
if it /is/ an enum class, not defining operator| is also acceptable. We may
want to tighten this up later.

No functionality change.

Swift SVN r30207