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

[Victor Lomuller via cfe-commits]

Naghasan wrote:

Still I forgot to answer this point as well...

> Are there ever point releases that might mea +ptx78 should actually expand to 780 rather than 78?

not sure what you exactly mean with your question. I guess we can mirror the cuda arch macro so, major * 100 + minor * 10, no opinion here. 

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