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

[Jan Voung via cfe-commits]

jvoung wrote:

Sorry for the delay!

I ran a few more tests and the downstream null analysis over a corpus of code with a small change to the patch. The small tweak (below) seems to pass the tests and your fix helps cover more code, as expected =)
(e.g., analyze both branches when there is an `if (enum1 == enum2)` vs only one branch)

I think the issue is that the downstream code doesn't expect to `nullptr_t` to have a pointer value / modeled nullability state until there is a cast (e.g., comments around https://github.com/google/crubit/blob/859520eca82d60a169fb85cdbf648c57d0a14a99/nullability/pointer_nullability_analysis.cc#L1130)

It would take more to change that assumption thoroughly.

So a suggestion is to:
- do the original fix to `evaluateBooleanEquality`
- to handle 'nullptr == nullptr' could special case that in `evaluateBooleanEquality` (e.g., check if LHS and RHS getType().isNullPtrType()) -- instead of creating PointerValues in VisitCXXNullPtrLiteralExpr

WDYT ?

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