swift234

The value witnesses for size/alignment/stride can just be size_t's.
This requires us to potentially copy the value witness tables for
generic struct types as part of computing layout, but that's not
the end of the world (although it will rely on a future patch
to split value witnesses out from protocol witness tables).

Oh, and add a value witness for stride, changing Builtin.strideof
to use that.

Swift SVN r2829