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

[Jan Voung via cfe-commits]

jvoung wrote:

Thanks, Martin, for the checking over the `nullptr`/`nullptr_t` modeling, and the suggestion for a `Value &getNullptrValue() const`! We can try that out and test it that as a followup cleanup for this special-casing.

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