[clang] [clang][SPIR-V] Add support for AMDGCN flavoured SPIRV (PR #89796)

[Alex Voicu via cfe-commits]

AlexVlx wrote:

> How much of this is actually different from the existing target info for AMDGCN? Seems like we're doing a lot of redundant stuff like defining macros or features.

That's part of the point, it's not actually supposed to differ in those particular regards, up to the point where things fork into specific GFXIPs. At the same time, there's no feasible way to re-use any of that, at least not one that I can see with how targets currently work. If you're suggesting that this should actually be based on AMDGPUTargetInfo, that's probably not the right way to go since that sets additional things that do not work with SPIRV at all.

https://github.com/llvm/llvm-project/pull/89796