Revert 172027 and 174336. Remove diagnostics about over-aligned stack objects.

Aside from the question of whether we report a warning or an error when we
can't satisfy a requested stack object alignment, the current implementation
of this is not good.  We're not providing any source location in the diagnostics
and the current warning is not connected to any warning group so you can't
control it.  We could improve the source location somewhat, but we can do a
much better job if this check is implemented in the front-end, so let's do that
instead.  <rdar://problem/13127907>