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

[via llvm-commits]

jeremyd2019 wrote:

Somewhat unrelated, I'm not seeing where the feature `case-insensitive-filesystem` is set.  It seems to be true on Cygwin, but Cygwin is not issuing the non-portable path warning on files that differ only in case, so that's probably actually a bug that I need to track down.

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