[clang] [clang][Driver][HIP] Add support for mixing AMDGCNSPIRV & concrete `offload-arch`s. (PR #113509)

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

> So, I'm thinking that SPIR-V here fits better as a triple. I've been wanting to generalize the `--offload` option to behave more like `-fopenmp-targets`. I.e. we would have `--offload=spirv64-amd-amdhsa,amdgcn-amd-amdhsa -Xarch_device_amdgcn-amd-amdhsa --offload-arch=gfx1030,gfx90a,gfx942` for a really verbose example. I'd like a separate option that simply targets the *toolchain* and then one that creates the `BoundArch`s. If it's not set then we default to some value (whatever amdgpu-arch says?). 

We don't use `offload` at the moment, that's for HIPSPV. Of course, future, fancy work is more than welcome, but this merely slots into the existing infra and current use cases.

https://github.com/llvm/llvm-project/pull/113509