[AutoDiff] Fix activity analysis use-after-free crash. (#28285)
Remove `DifferentiableActivityInfo::getLookupConformanceFunction`, which
returned a `LookupConformanceFn` (type alias for `LookupConformanceFn`),
which does not own the underlying callable.
Add a `DifferentiableActivityInfo::hasTangentSpace` helper, which is easier
to use and avoids the issue.
Resolves TF-945.
