swift234

Runtime: Add swift_allocBox.

Add a runtime function that, given a generic type metadata pointer, allocates a heap object capable of containing a value of that type. This is a first-pass implementation that always does the worst case thing of stuffing the type metadata into the box with the value and using its value witness table to size, align, and destroy the box. Use swift_allocBox to implement ArchetypeTypeInfo::allocate correctly for heap object allocations. This means SIL's alloc_box $T now works for archetypes, and a simple generics test now (almost) compiles through SIL!

Swift SVN r4599