[clang][dataflow] Use `IntegerValue` instead of `StructValue` in `ValueTest`.

Soon, it will no longer be possible to default-construct `StructValue`. For details, see https://discourse.llvm.org/t/70086.

For completeness, also add a test that `areEquivalentValues()` on different `IntegerValue`s returns false.

Reviewed By: xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D154934