[llvm] [WindowsDriver] Always consider `WinSdkVersion` (PR #130377)

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

> Do we have any tests for these codepaths, and if not, would it be feasible to add some?

I think that I convinced myself that there is no good way to test this. The reason is that we want to honour `-Xmicrosoft-windows-sdk-version` without `-Xmicrosoft-windows-sdk-root`. When doing so, we would rely on the system lookup. Testing without `-Xmicrosoft-windows-sdk-root` implies that we would need to ensure that the machine has a very specific shape and makes the tests non-portable. It is unclear to me if that is worth it.

https://github.com/llvm/llvm-project/pull/130377