swift234

IRGen: Implementation of typed box operations.

When producing TypeInfo for a box, try to reuse instantiations for common type structures:

- any POD type with the same stride and alignment can share a fixed HeapLayout;
- any single-refcounted-pointer type can share a fixed HeapLayout with types that have the same ReferenceCounting;
- dynamically-sized types can share a runtime-based box implementation.

For the runtime implementation, use new to-be-implemented variants of allocBox/deallocBox that will produce polymorphically-projectable boxes using instantiated metadata.

Swift SVN r29612