[libcxx] Convert paths to/from the right narrow code page for narrow strings on windows

On windows, the narrow, char based paths normally don't use utf8, but
can use many different native code pages, and this is what system
functions that operate on files, taking such paths/file names, interpret
them as.

Differential Revision: https://reviews.llvm.org/D91137

GitOrigin-RevId: de698ae73444b5160dd6b8d768b30d6764be004e
3 files changed