Per John's comments, make a GenericBox struct to represent the layout of a generic box allocation, move all the layout calculations into methods on GenericBox, and alter swift_deallocBox to take the type as an argument so that it's future-proofed for when we start special-casing certain types in allocBox.
Swift SVN r4613
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
Previously, we were relying on objc_retain to retain the closure, but
closure contexts aren't actually id-compatible. Now we use a C++ RAII
object for Swift to manage the closure context retain count.
In the process, Joe and I discovered that the closure context was being
passed to the conversion shim at +0; that has now been fixed to be +1.
Swift SVN r3948
