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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> I looked at this and it doesn't look super appealing (turned into something of a rabbit hole), it'd duplicate a lot of the existing toolchain, and would also try to squat in an already overcrowded space (there's already HIPSPV).

We already have a SPIR-V toolchain for HIP? Seems like it was added over two years ago. What does the other handling do that this toolchain doesn't?

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