[libc++] Try to fix cross-loading of lit.local.cfg on Windows

[libc++] Try to fix cross-loading of lit.local.cfg on Windows

On windows, the previous path replacement using forward slashes wouldn't
work, and so we'd end up including the same file again. We would do that
until we'd hit the recursion limit of the Python interpreter.

Instead, use `os.path` to properly replace without assuming a specific
path separator.