[clang] [HLSL] update default validator version to 1.8. (PR #104040)

[Damyan Pepper via cfe-commits]

damyanp wrote:

> > How does this relate to #102963?
> 
> It will make default 1.8 so default case will work. But we still need to have a plan for what to do with -validator-version.

Chatting with @bharadwajy, it sounds like this PR should also fix #102963.

https://github.com/llvm/llvm-project/pull/104040