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

[Alex Voicu via cfe-commits]

AlexVlx wrote:

> `__constant__` may not necessarily be `const` for IR purposes. I.e. IR may not rely on the 'known' values, as seen in IR, as the data may actually be populated by the host via CUDA API calls `cudaMemcpyToSymbol` before the GPU kernel launch.

But since this is marked `externally_initialised` on the device side, one would assume that those semantics still hold i.e. there's no known value to assume?

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