[lit] Implement "-r" option for builtin "diff" command + a test using that.

Summary:
That would allow to recursively compare directories in tests using
"diff -r" on Windows in a similar way as it can be done on Linux or Mac.

Reviewers: zturner, morehouse, vsk

Reviewed By: zturner

Subscribers: kcc, llvm-commits

Differential Revision: https://reviews.llvm.org/D41776

llvm-svn: 322102