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

[Deric C. via llvm-commits]

Icohedron wrote:

No, the PRs are not blocking each other. They are both supposed to reflect how DXC works as currently written in the source code.
The update to the DXIL.rst docs in DXC is an NFC since it is just documentation.

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