Fix crash when emitting error.

With templated classes, is possible to not be able to determine is a member
function is a special member function before the class is instantiated.  Only
these special member functions can be defaulted.  In some cases, knowing
whether a function is a special member function can't be determined until
instantiation, so an uninstantiated function could possibly be defaulted too.
Add a case to the error diagnostic when the function marked with a default is
not known to be a special member function.

llvm-svn: 282989