swift234

IRGen: Fix stupid oversight in emission of fixed multi-payload enum vw_getEnumTag()

The payload tag discriminates between payload cases, but empty cases
are stored in the common spare bits of the payload types, so the logic
here would report all empty cases as having the first empty case selected.
And when writing tests, I didn't cover enums with multiple empty *and*
non-empty cases. Oops...

Now we emit a little bit more code to assemble the correct case index
from both the payload tag and payload value, then select between the
two values depending on the payload tag.

Fixes <rdar://problem/22192074>.

Swift SVN r31328