[clang] Add SPIRV support to HIPAMD toolchain (PR #75357)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Perhaps we should consider prefixing it in some way (e.g. `hip-spirv` or `amd-spirv`) that leaves the door open for some special handling (enable a particular set of extensions only for amdgpu targeting SPIRV, try to deal with missing builtins etc.) / flexibility?

Unsure that's necessary, as we'd already have `OFK_HIP` in the clang driver or `-x hip` in `cci` to key off of if needed.

https://github.com/llvm/llvm-project/pull/75357