[CUDA/HIP] Externalize __device__ const variables accessed by host code (#177292) In standard C++, const variables at namespace scope have internal linkage. For __device__ const variables, this makes them invisible to runtime symbol lookup APIs (cudaGetSymbolAddress/hipGetSymbolAddress). Reading a __device__ const variable from host code is a valid usage pattern — the host may need to know the value at runtime. This is also needed by libcudacxx's cuda::get_device_address. This patch extends the existing CUDADeviceVarODRUsedByHost tracking to cover __device__ const variables. When host code references such a variable, it gets externalized (same mechanism used for static device vars). Variables only used in device code keep internal linkage and can still be constant-folded. The fix is in SemaExpr: __device__ const variables are classified as CVT_Both (due to an implicit CUDAConstantAttr), so the ODR-use tracking is extended to include CVT_Both variables with an explicit CUDADeviceAttr, distinguishing them from plain const variables.
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