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

[Alexandre Ganea via llvm-commits]

aganea wrote:

@Sharjeel-Khan Are you able to check if applying https://github.com/aganea/llvm-project/commit/606253ef25653cb06c59d4d53197a626a6a2e272 on ToT fixes the issue you're seeing?

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