Do not give implicitly-defined virtual members functions

available_externally linkage, since they may not have been given a
strong definition in another translation unit. Without this patch, the
following test case fails to link with a GCC-compiled libstdc++:

  #include <sstream>
  int main() { std::basic_stringbuf<char> bs; }

Fixes the last problem with the Boost.IO library.

llvm-svn: 103208