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

[Martin Storsjö via llvm-commits]

mstorsjo 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?

For the case of mingw headers, these defines should be available since https://github.com/mingw-w64/mingw-w64/commit/3a137bd87ea83d174f4a225568b75cabdbc70852, available in mingw-w64 v12 and v13.

I'm not sure if we strictly want to require any specific minimum SDK to be used here - perhaps this can go within an `#ifdef THREAD_POWER_THROTTLING_EXECUTION_SPEED` which indicates that a new enough SDK is available? (I checked that this constant is provided as a `#define`, as opposed to an enum, in WinSDK too.)

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