Swift class metadata has a bit to distinguish it from non-Swift Objective-C
classes. The stable ABI will use a different bit so that stable Swift and
pre-stable Swift can be distinguished from each other.
No bits are actually changed yet. Enabling the new bit needs to wait for
other coordination such as libobjc.
rdar://35767811
This way they can be used from other projects, like LLDB. The downside
is we now have to make sure the header is included consistently in all
the places we care about, but I think in practice that won't be a problem,
especially not with tests.
rdar://problem/22240127
Swift SVN r31173
