[clang] [HLSL] Make fast math the default for HLSL (PR #119820)

[Justin Bogner via cfe-commits]

bogner wrote:

> Two questions:
> 
> 1. Do we foresee any issue adding `afn` (I suspect not)

Adding `afn` should be fine - we don't have library functions per se anyway so this shouldn't make any difference one way or the other.

> 2. Do we also need to apply `contract`, which I believe was also implied by `fast` in 3.7 although the docs are unclear?

Less clear, but I think we should err on the side of omitting it for now. We can reevaluate when we're comparing more complex shaders between clang and dxc.
 
> Also, I think we may need to put implement the backend support for translating the fast math flags to 3.7-equivalents before we merge this. I'm sorry I hadn't thought of that until now, but I expect this change breaks DXIL generation.

This is a good point. We definitely need to hold off on putting this in for now - the bitcodewriter actually crashes on these flags currently. Can you make sure something is filed to track that?

https://github.com/llvm/llvm-project/pull/119820