When storing a tag into a multi-payload enum for an empty case via
swift_storeEnumTagMultiPayload, the tag is split between the storage for
payloads (i.e. the associated values of the non-empty cases) and the
storage for tags (i.e. the integers which refer to the non-empty cases).
Typically, the number of non-empty cases (i.e. one beyond the tag
that refers to the last non-empty case) is stored into the tag storage
and the integer which distinguishes which among the empty cases the enum
is in is stored into storage for payloads. When the enum is small,
however--specifically, when the payload size is less than four
bytes--that information is packaged differently via masking.
Previously, there was a problem with that masking. While the value
stored into the tag storage was correct (and correctly indicated that
the enum was in some empty case), the value stored into the payload
storage was not. The result was that which empty case an enum was in
would be corrupted.
Here, that is fixed by fixing the masking.
rdar://87914343
