[llvm] [DirectX] Set Shader Flag DisableOptimizations. (PR #123136)

[Justin Bogner via llvm-commits]

bogner wrote:

Is the `dx.disable_optimizations` metadata documented somewhere? This all looks like it will work, but I don't really understand why we have a custom metadata node coming from the frontend here rather than just looking at `llvm::Attribute::OptimizeNone` on the entry function or something like that.

https://github.com/llvm/llvm-project/pull/123136