[CUDA][HIP] Do not mark extern shared var (#65990)

Fixes: https://github.com/llvm/llvm-project/issues/65806

Currently clang put extern shared var ODR-used by host device functions
in global var __clang_gpu_used_external. This behavior was due to
https://reviews.llvm.org/D123441. However, clang should not do that for
extern shared vars since their addresses are per warp, therefore cannot
be accessed by host code.