swift234

Change the layout of opaque existentials so that
the value buffer comes first.

The motivation for doing this is similar to the
motivation for moving it for class existentials:
it eliminates the need for an offset for the most
common accesses, which is particularly important
for the generic value witnesses.

Also try to hard-code that layout in fewer places,
or at least static_assert the places that have to
do so.

Swift SVN r16279