[AutoDiff] Better diagnose non-differentiability for throwing functions

[asl]

Fixes #81608

[asl]

@swift-ci please test

[kovdan01]

As far as I see, we now have 3 new code paths leading to non-differentiability diagnostics, while only 1 is covered by test. It's probably worth adding tests for all the 3 code paths (if that's not too challenging)

Except this, LGTM

[asl]

As far as I see, we now have 3 new code paths leading to non-differentiability diagnostics, while only 1 is covered by test. It's probably worth adding tests for all the 3 code paths (if that's not too challenging)

Except this, LGTM

In fact two, as currently there is no way getSingleTerminator would return nullptr. Still, if new terminators would be added in the future it would be better not to crash here. And as for move_value we previously decided with you to ignore that case :)