[clang] [clang][dataflow] Fix unsupported types always being equal (PR #129502)

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Discookie wrote:

Sounds good to me, thanks for investigating! Opened #131575 with the recommended change.

In the future, it would be nice indeed for nullptr literals to have some kind of Value - I also encountered this as a pain point in my own experimental checker. But with the presence of smart pointers, it indeed needs more thought than this simple fix.

https://github.com/llvm/llvm-project/pull/129502