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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Thanks for the update - I’ll try it on an older windows later.

You’ll need to update WindowsSupport.h with the changes from my linked commit https://github.com/mstorsjo/llvm-project/commit/windows-rename-fix (and merge the latest changes from main first) for this to compile on mingw targets, whichever way we go with the fallback.

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