[clang] [llvm] [cuda][HIP] `__constant__` should imply constant (PR #110182)

[Alex Voicu via cfe-commits]

AlexVlx wrote:

I've extende the test under `Transforms/GlobalOpt/externally-initialized.ll` to also cover `constant`s / ensure they don't get CSEd.

https://github.com/llvm/llvm-project/pull/110182