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