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

[Alex Voicu via cfe-commits]

AlexVlx wrote:

@yxsamliu beat me to it. The problem here is that the example illustrates language level const on a variable with internal linkage (which is mandatory for `__constant__` variables in non-RDC compilation AFAICT from 17.5.24.1) and we don't set externally_initialized on that, which matches C/C++ expected semantics, I believe (there's an oblique mention as to the reasoning, which is that the loader should be allowed to place these in `.rodata` if they materialise at all). Regarding the lit test, I can definitely add another one, but I will note we already have `Transforms/GlobalOpt/externally-initialized.ll`, which probably can be extended?

Please note that this is something that wouldn't work (as apparently expected) today either. The user can write the code  @Artem-B wrote (put const on a `__constant__`, add an initialiser because otherwise things would be broken), and they'd get exactly the potentially unexpected behaviour. The very fact that the godbolt example exists points that out (the example also helpfully points out that NVCC has the same behaviour). Unfortunately the CUDA spec does not appear to talk about this (a `const` qualified `__constant__` variable) directly, but working back from 17.5.23.2 we can conclude that they're fine as long as the host compiler is not VC++.

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