A recent change made accessibility checking stricter. This had some
fallout on the half-baked @_versioned attribute, where we could no
longer define @_versioned members on a non-@_versioned type.
This was wrong anyway (and will be diagnosed when we add proper
diagnostics for @_versioned), because type metadata for the
internal type did not get the right linkage, but it used to work
as long as you didn't try to get the type metadata at runtime.
This patch adds @_versioned attributes to the right types now that
this broken behavior is gone.
As a result, _Variant{Set,Dictionary}Storage became resilient
(non-@_versioned internal types are not resilient), which broke
too many tests that assumed you can exhaustively switch over all
the cases. Since eager-bridging is going to eliminate this enum
anyway (or so I've heard), make it @_fixed_layout for now.
From the Swift documentation:
"If you define an optional variable without providing a default value,
the variable is automatically set to nil for you."
