[lit] Compare to None using identity, not equality

Summary:
In Python, `None` is a singleton, so checking whether a variable is
`None` may be done with `is` or `is not`. This has a slight advantage
over equiality comparisons `== None` and `!= None`, since `__eq__` may
be overridden in Python to produce sometimes unexpected results.

Using `is None` and `is not None` is also recommended practice in
https://www.python.org/dev/peps/pep-0008:

> Comparisons to singletons like `None` should always be done with `is` or
> `is not`, never the equality operators.

Patch by Brian Gesiak!

Reviewers: ddunbar, echristo, beanz

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D25168

llvm-svn: 283088