[clang] [llvm] [CIR][CUDA] Shadow variables, internalize device side variables, lower poison attribute (PR #190087)

[David Rivera via cfe-commits]

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RiverDave wrote:

Could we add a CIRGen test similar to `clang/test/CIR/CodeGen/HIP/global-vars.cpp` from llvm/clangir#2111? 

That file covers the relevant attribute variations and would give us confidence that we emit the right flags.

https://github.com/llvm/llvm-project/pull/190087