[clang] [Clang][Cygwin] Enable few conditions that are shared with MinGW (PR #149637)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

Also, if we could easily cover some of these cases with tests, that'd be nice.

I'd recommend rewriting the co-authored line in the PR description into something which isn't an `@username` reference, as that produces large amounts of notifications when the commit travels downstream into various repos - see https://discourse.llvm.org/t/forbidding-username-in-commits/86997.

https://github.com/llvm/llvm-project/pull/149637