Make use of available_externally linkage for vtables when the

non-inline key function of a class template instantiation, when no key
function is present, the class template instantiation itself was
instantiated with an explicit instantiation declaration (aka extern
template). I'm fairly certain that the C++0x specification gives us
this lattitude, although GCC doesn't take advantage of it.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@92779 91177308-0d34-0410-b5e6-96231b3b80d8