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

[Alexandre Ganea via llvm-commits]

aganea wrote:

I made the change to not use `TEXT()` as you suggested; and also moved the function to `WindowsSupport.h` since I'd like to use in other places that retrive system modules directly without their full path qualification.

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