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

[via cfe-commits]

mmoadeli wrote:

> It would be great to add some tests for local AS null pointers for NVPTX and AMDGPU back-ends.

They already have it [here](https://github.com/llvm/llvm-project/blob/286ac8f3ea6aec711827ccab9608b010e78b18cf/clang/test/CodeGenSYCL/amd-address-space-conversions.cpp#L24)

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