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

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jeremyd2019 wrote:

@mati865  this is what was left of your patch, plus a couple more places I found grepping for WindowsGNU and considering whether the cases would also apply to Cygwin.  I did back out the change you had to `WinX86_64ABIInfo` because that ABIInfo is not used on Cygwin (Cygwin has its own ABIInfo class)

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