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

[Artem Belevich via cfe-commits]

Artem-B wrote:

It has nothing to do with writing to those arrays while the kernel is running. That would indeed be UB.

> both would still work just the same even with this change,

No, they will not. Here's the demonstration of the behavior change that `const` brings to the table:
https://cuda.godbolt.org/z/h6e5Wb4PT

It shows that adding `const` allows compiler to use the initializer values as written (it does not even bother instantiating `cxxx`), while without `const` it reads the array values, assuming that they may be changed externally.


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