LWG issues 2174/5/6 - mark wstring_convert::converted as noexcept, and make (some of) the constructors for wstring_convert and wbuffer_convert as explicit. Add configuration macro _LIBCPP_EXPLICIT_AFTER_CXX11

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189398 91177308-0d34-0410-b5e6-96231b3b80d8