[llvm] Windows: use EcoQoS for ThreadPriority::Background (PR #148797)

[Tim Blechmann via llvm-commits]

timblechmann wrote:

hmm, they should come from processthreadsapi.h (via windows.h). makes me wonder if it requires a minimum requirement needs to be defined explicitly?

https://github.com/llvm/llvm-project/pull/148797