[lit] Make internal diff work in pipelines

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 -
 # RUN: not diff file1 file2 | FileCheck %s
```

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` cannot
currently be used in pipelines and doesn't recognize `-` as a
command-line option.

To enable pipelines, this patch moves lit's `diff` implementation into
an out-of-process script, similar to lit's `cat` implementation.  A
follow-up patch will implement `-` to mean stdin.

Reviewed By: probinson, stella.stamenova

Differential Revision: https://reviews.llvm.org/D66574

llvm-svn: 372035