swift234

Add Builtin.UnsafeValueBuffer, which provides opaque
storage for arbitrary values.

A buffer doesn't provide any way to identify the type of
value it stores, and so it cannot be copied, moved, or
destroyed independently; thus it's not available as a
first-class type in Swift, which is why I've labelled
it Unsafe.  But it does allow an efficient means of
opaquely preserving information between two cooperating
functions.  This will be useful for the adjustments I
need to make to materializeForSet to support safe
addressors.

I considered making this a SIL type category instead,
like $@value_buffer T.  This is an attractive idea because
it's generally better-typed.  The disadvantages are that:
- it would need its own address_to_pointer equivalents and
- alloc_stack doesn't know what type will be stored in
  any particular buffer, so there still needs to be
  something opaque.

This representation is a bit gross, but it'll do.

Swift SVN r23903