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

[Fabrice de Gans via llvm-commits]

Steelskin wrote:

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

As @compnerd mentioned, this is relying on the installation environment. Does LLVM have test facilities for faking the registry and the file system? Otherwise, this is going to require a refactor of this file so we can inject a fake Windows SDK installation path for testing purposes.

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