swift234

Reflection: Emit metadata for fixed-layout SIL boxes

We were recovering metadata from generic boxes by reading
the instantiated payload metadata from the box's metadata,
but this approach doesn't work for fixed-size boxes, whose
metadata does not store the payload metadata at all.

Instead, emit a capture descriptor with no metadata sources
and a single capture, using the lowered AST type appearing
in the alloc_box instruction that emitted the box.

Since box metadata is shared by all POD types of the same
size, and all single-retainable pointer payloads, the
AST type might not accurately reflect what is actually in
the box.

However, this type is *layout compatible* with the box
payload, at least enough to know where the retainable
pointers are, because after all IRGen uses this type to
synthesize the destructor.

Fixes <rdar://problem/26314060>.