[clang] [llvm] [clang][docs] Add preliminary documentation for SPIR-V support in the HIPAMD ToolChain (PR #96657)

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

> I'll need to play with this with my driver code. I'm guessing it's because it needs to generate an entirely separate toolchain? The OpenMP path basically does that by inferring the toolchain from the string value, so we can support `--offload-arch=sm_89,gfx90a` for example. 

Not quite, it's more because we'd have to nest two triples (`spirv64-amd-amdhsa` && `amdgcn-amd-amdhsa`) within the same toolchain, since we're using the same HIPAMD ToolChain. It's fixable, just slightly faffy to do without spamming toolchains / within the same toolchain.

https://github.com/llvm/llvm-project/pull/96657