Emit debug info for trait object pointer

Emit better debugging information for a trait object pointer.  In
particular, now:

* The fields are explicitly represented in the DWARF;
* DWARF for the vtable itself is emitted; and
* The DWARF for the vtable's type has a DW_AT_containing_type which
  points to the concrete type for which the vtable was emitted.  This is
  a small DWARF extension, that allows debuggers to determine the real
  type of the object to which a trait object points.

I'll submit the gdb patch to take advantage of this new debuginfo once
this lands.

The vtable type is not currently complete -- it doesn't include members
for the pointers it contains.  This information was not needed for this
feature.

This addresses part 1 of #1563.

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