swift234

Runtime: Add swift_getExistentialMetadata entry point.

Set up a metadata cache for existential type metadata. Instantiate existential metadata by first sorting the protocol list, so that it is order invariant, precomputing the overall witness table count and class constraint of the composition so it can be cached in the existential metadata.

We still need to implement value witnesses for existential containers in the runtime before this is complete. We can at least test the uniquing and flags computations at this point.

Swift SVN r9727