[llvm] [DirectX] Fix shader flag version-checking logic to match DXC (PR #136787)

[Finn Plummer via llvm-commits]

https://github.com/inbelic approved this pull request.

IIUC much of the logic defined here is derived from DXC and is meant to match that.

This LGTM in the sense that the logic implemented matches the behaviour described in the PR description, which I believe is what we should be basing it off of.

https://github.com/llvm/llvm-project/pull/136787