Re-export two previously exported std::string functions.

These functions were removed from the dylib sometime between the 3.9 release
and now. This patch manually exports them to re-gain ABI compatibility.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@284193 91177308-0d34-0410-b5e6-96231b3b80d8