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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Our buildbot pulls the new LLVM commits and rebuilds every 6 hours so I'll just wait for the next run which is at 12 pm

He pointed to his private fork - his suggested change isn't included in the latest git main, so it won't be automatically tested by your buildbot.

Can you check the version of `__MINGW64_VERSION_MAJOR` in your `_mingw_mac.h` header?

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