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

[Artem Belevich via cfe-commits]

Artem-B 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.

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