[clang][dataflow] Don't propagate result objects in unevaluated contexts (#90438)

Trying to do so can cause crashes -- see newly added test and the
comments in
the fix.

We're starting to see a repeating pattern here: We're getting crashes
because
`ResultObjectVisitor` and `getReferencedDecls()` don't agree on which
parts of
the AST to visit and, hence, which fields should be modeled.

I think we should ensure consistency between these two parts of the code
by
using a `RecursiveASTVisitor` in `getReferencedDecls()`[^1]; the
`Traverse...()` functions that control which parts of the AST we visit
would go
in a common base class that would be used for both `ResultObjectVisitor`
and
`getReferencedDecls()`.

I'd like to focus this PR, however, on a targeted fix for the current
crash and
postpone the refactoring to a later PR (which will be easier to revert
if there
are unintended side-effects).

[^1]: As an added bonus, this would make the code better structured and
more
efficient than the current sequence of `if (dyn_cast<T>(...))`
statements).