[clang] [Clang] Permit `vulkan` as an OS for the SPIRV target architecture (PR #196101)

[Dmitry Sidorov via cfe-commits]

https://github.com/MrSidims commented:

The change looks good to me, with one note: per my understanding 'vulkan', 'vulkan1.2' and 'vulkan1.3' are effectively all different OSes from target triple perspective. If my understanding is right - this should be addressed.

https://github.com/llvm/llvm-project/pull/196101