[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
llvm-svn: 375116