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

[via llvm-commits]

jeremyd2019 wrote:

A couple of these tests seemed like they might make more sense as a target rather than system check, but I followed what they were already doing.  Also, confusingly, llvm/utils/lit/tests/lit.cfg includes cygwin with `system-windows`: https://github.com/llvm/llvm-project/blob/dddeb07c2ea9bc4e507d3bd34980fa6e9513ed9f/llvm/utils/lit/tests/lit.cfg#L121-L123

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