Do not give implicitly-defined virtual members functions

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103208 91177308-0d34-0410-b5e6-96231b3b80d8