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

[Artem Belevich via cfe-commits]

Artem-B wrote:

> In this case, _ZL4cxxx does not have externally_initialized . If this patch does not remove externally_initialized, probably this constant propagation won't happen.

Indeed, unoptimized code shows that `cxxx` has no `externally_initialized`, only `constant`. 

If we keep externally_initialized, it appears to preserve the array and the loads from it, so it should be fine: https://godbolt.org/z/YeWE3z79r


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