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

[Sharjeel Khan via llvm-commits]

Sharjeel-Khan wrote:

> 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.

I noticed after my message it was a private fork change. I began testing it locally.

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

It is 7

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