Add Support for Creating and Deleting Unicode Files and Directories in Lit

This enables lit to work with unicode file names via mkdir, rm, and redirection.
Lit still uses utf-8 internally, but converts to utf-16 on Windows, or just utf-8
bytes on everything else.

Committed on behalf of Jason Mittertreiner
Differential Revision: https://reviews.llvm.org/D56754

llvm-svn: 355122