[test] Add lit helper for windows paths

This adds 2 new lit helpers `%{fs-src-root}` and `%{fs-sep}`, these
allow writing tests that correctly handle slashes on Windows. In the
case of tests like clang/test/CodeGen/debug-prefix-map.c, these are
unable to correctly test behavior on both platforms, unless they fork
and add OS requirements, because the relevant logic hits host specific
codepaths like checking if paths are absolute.

Differential Revision: https://reviews.llvm.org/D111457