Add Shader Model 6.9 to main

[llvm-beanz]

We are beginning development of SM 6.9 and working with partners. To facilitate this we should add the shader model support to main.

[damyanp]

Need to also figure out validator / hashing to support PREVIEW appropriately.

[damyanp]

Also consider making it so that preview / incomplete features don't appear in the help text, or something like that to make it clear that these features aren't actually ready for production use yet.

[bob80905]

Closed by #7075