Debuginfo "extended instruction sets" (e.g. `OpenCL.DebugInfo.100`, `NonSemantic.Shader.DebugInfo.100`).

[eddyb]

Right now we only support the SPIR-V instructions OpLine/OpNoLine, Rust-GPU does its own custom instructions to add a bit more information, and there's also SPIR-T-only attributes:

• Replace Attr::SpvDebugLine with a better basic debug source location attribute. #9
  (the intended purpose of the newer attribute is to have Rust-GPU generate it for internal use)

Ideally, though, we could interop with SPIR-V's equivalents of DWARF:

• OpenCL.DebugInfo.100
• NonSemantic.Shader.DebugInfo.100

Sadly, they each have drawbacks/limitations, so we probably need to do a tiered system where we can express all of their metadata (e.g. as weirdly-typed Consts) and only convert some of them into SPIR-T-native attributes etc. - at the very least, we should be able to do correct passthru even of features that Rust-GPU itself doesn't use/need.