[lit] Extend internal diff to support `-` argument

When using lit's internal shell, RUN lines like the following
accidentally execute an external `diff` instead of lit's internal
`diff`:

```
 # RUN: program | diff file -
```

Such cases exist now, in `clang/test/Analysis` for example.  We are
preparing patches to ensure lit's internal `diff` is called in such
cases, which will then fail because lit's internal `diff` doesn't
recognize `-` as a command-line option.  This patch adds support for
`-` to mean stdin.

Reviewed By: probinson, rnk

Differential Revision: https://reviews.llvm.org/D67643