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