Encode testing information in the comment/header for HLSL

[kvark]

In gfx-rs/naga#972 @Indish laid out the infrastructure for validating HLSL. It uses special "*.config" files on the sideline of the HLSL output. It was necessary, I'm happy with the way it works.

Now I had a similar issue with MSL where I needed to tell the validator, which language version to validate against. And I decided to try to encode this just as a comment in the file header. See - gfx-rs/naga#1101 . I think this works very well, and we should consider doing the same for HLSL. Here is what we'll get in return:

• no need to have separate "*.config" files
• no redundancy: the backend already knows what to put into this comment, no human intervention needed