[lit] [windows] Make sure to convert all path separators to backslashes in NT style \\?\... paths

E.g. the mingw python distributed in msys2 (the mingw one, which is a
normal win32 application and doesn't use the msys2 runtime itself),
despite being a normal win32 python, still uses forward slashes. This
works fine for other cases (many, but not all), but when constructing a
raw NT path, all path separators must be backslashes.

Differential Revision: https://reviews.llvm.org/D71490