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

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macurtis-amd wrote:

> > Kernels produced by ROCm classic flang may have both nvvm "kernel" annotation and amdgpu_kernel calling convention.
> 
> I think this should be fixed in classic flang instead of here. This is treating symptoms instead of root cause.

Also, it wasn't clear to me that this was invalid. That is, I wasn't sure if the presence of the nvvm "kernel" annotation always implied a PTX kernel. Documentation [here](https://docs.nvidia.com/cuda/nvvm-ir-spec/index.html#global-property-annotation-chapter-11) seems to imply that the annotations may be used in other cases.

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