[clang] [clang][NVPTX] Define macro indicating the PTX version (PR #94934)

[via cfe-commits]

ldrumm wrote:

Can you please include rationale for why this this name e.g. why not `__NVPTX_VERSION__`?
How does it relate to `__CUDA_ARCH__`, and why is `__CUDA_ARCH__` not sufficient?
Are there ever point releases that might mean we should max +ptx78 should actually expand to `780` rather than `78`?

Once a macro is defined - that's *it* ; it's there forever - so I'd like a bit more clarity on the decisions making even if only in your commit message 



https://github.com/llvm/llvm-project/pull/94934