This patch fixes a warning that is issued when -Wpadded is on clang's command line. The following warning is from the build log of http://lab.llvm.org:8080/green/view/Libcxx/job/libcxx_abi/554/consoleText (note that I see this warning issued only from builders that use lib/buildit):
In file included from ../src/stdexcept.cpp:10:
../include/__refstring:30:12: warning: padding size of 'std::1::libcpp_refstring::_Rep_base' with 4 bytes to alignment boundary [-Wpadded]
struct _Rep_base ^
1 warning generated.