libcxx: Provide overloads for basic_filebuf::open() et al that take wchar_t* filenames on Windows.

This is an MSVC standard library extension. It seems like a reasonable
enough extension to me because wchar_t* is the native format for
filenames on that platform.

Differential Revision: https://reviews.llvm.org/D42225

llvm-svn: 323170