[llvm] Support: unlock Windows API support, switch to Windows 10 RS1+ APIs (PR #102240)

[via llvm-commits]

jeremyd2019 wrote:

> Anyway, the amount of APIs of this kind that we use/need is pretty small, so I don’t see a huge benefit in bumping the requirement (and in this case, making it fallback is quite easy). (A far more tricky aspect in these codepaths, is making them work properly with various less common file system drivers, like third party ramdisks and network mounts and similar.)

I think Cygwin has a whole pile of fallback and workaround cases for posix semantics due to filesystem drivers, so I think having a fallback is going to be essential regardless of minimum OS version support.  Example: https://github.com/cygwin/cygwin/commit/fe2545e9faaf4bf9586f61a7b83d5cb5af501194

https://github.com/llvm/llvm-project/pull/102240