[clang] [NVPTX][AMDGPU][CodeGen] Fix `local_space nullptr` handling for NVPTX and local/private `nullptr` value for AMDGPU. (PR #78759)

[Artem Belevich via cfe-commits]

Artem-B wrote:

> * Address space cast of nullptr in local_space into a generic_space for the CUDA backend.

I think you mean "NVPTX back-end". CUDA is a front-end entity (C++ w/ GPU extensions)

https://github.com/llvm/llvm-project/pull/78759