swift234

Runtime: Fix alignment issue in default witness tables

The size of a protocol's metadata was not a multiple of 8 bytes, so
on 64-bit platforms, the runtime would copy default witnesses from
the wrong address, because IRGen metadata does not add alignment padding,
whereas the in-memory structure does.

Fix this by adding a 32-bit padding field at the end of the protocol
descriptor. Technically this is not necessary on 32-bit, but this keeps
things simpler for now.

The test case for this is a library evolution test exercising resilient
default protocol requirements, but it is not quite ready to go in yet.