Let cxa_demangle.cpp compile with gcc/libstdc++ 4.8 and clang-cl/MSVC2013's STL.

libstdc++ needs a few typedefs in malloc_alloc. MSVC's STL needs rebind(),
construct(), destroy().  MSVC2013 also has no snprintf, but it exists in 2015.

llvm-svn: 248129