[clang] [llvm] [Tests] Add system-cygwin feature, and use it. (PR #152611)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> I'm thinking it might be better to separate into 2 PRs, one just changing lit/llvm/config.py and another with all the test changes?

That sounds ok to me too.

https://github.com/llvm/llvm-project/pull/152611