[clang][dataflow] Crash fix for `widenDistinctValues()`. (#89895)

We used to crash if the previous iteration contained a `BoolValue` and
the
current iteration contained an `IntegerValue`. The accompanying test
sets up
this situation -- see comments there for details.

While I'm here, clean up the tests for integral casts to use the test
helpers we
have available now. I was looking at these tests to understand how we
handle
integral casts, and the test helpers make the tests easier to read.