[boost] [config] Patch for clang.hpp to support clang in Windows using VC++ RTL

Once Boost 1.55 is out the doors can I apply the attached patch for clang when _MSC_VER is defined ? Unfortunately clang in this situation does not currently support the C++11 types char16_t or char32_t ( there is a bug report for this ), and this means trying to compile Boost libraries like type_traits without this patch yields many errors. I would have used the BOOST_WORKAROUND macro but clang does not support a value which gives a single number for the compiler version so I felt that drumming up multiple BOOST_WORKAROUNDs using major/minor/patch version numbers was too much of a hack to bother about.