[llvm] [llvm] Preserve existing calling conv when auto-upgrading nvvm.annota… (PR #125568)

[Shilei Tian via llvm-commits]

shiltian wrote:

If I understand correctly, classical fortran is deprecated or being deprecated. There is no much point supporting it in current trunk for backward compatibility.

If we really don't want to break existing bitcode, I think it makes more sense to add this in our own fork.

https://github.com/llvm/llvm-project/pull/125568