[AutoDiff] Run AutoDiff closure spec pass for all VJPs

[kovdan01]

Previously, AutoDiff closure specialization pass was triggered only on VJPs containing single basic block. However, the pass logic allows running on arbitrary VJPs. This PR enables the pass for all VJPs unconditionally. So, if the pullback corresponding to multiple-BB VJP accepts some closures directly as arguments, these closures might become specialized by the pass. Closures passed via payload of branch tracing enum are not specialized - this is subject for future changes.

The PR contains several commits.

1. The thing named "call site" in the code is partial_apply of pullback corresponding to the VJP. This might appear only once, so we drop support for multiple "call sites".
2. Enhance existing SILOptimizer tests for the pass.
3. Add validation-tests for single basic block case.
4. The change itself - delete check against single basic block.
5. Add validation-tests for multiple basic block case.
6. Add SILOptimizer tests for multiple basic block case.

[kovdan01]

Tagging @JaapWijnen

@clackary Could you please run your tests against the toolchain built from mainline with this PR applied?

[kovdan01]

@swift-ci please test

[kovdan01]

@swift-ci please test

[kovdan01]

@swift-ci please test

[kovdan01]

@swift-ci please test

[kovdan01]

preset=buildbot,tools=RA,stdlib=DA
@swift-ci please test with preset

[clackary]

@kovdan01 I cherrypicked this onto swift-DEVELOPMENT-SNAPSHOT-2025-05-14-a tag, built a toolchain, and can compile all our internal projects that import _Differentiation in release mode. Looks good from that perspective.

[kovdan01]

@kovdan01 I cherrypicked this onto swift-DEVELOPMENT-SNAPSHOT-2025-05-14-a tag, built a toolchain, and can compile all our internal projects that import _Differentiation in release mode. Looks good from that perspective.

@clackary Thanks! Could you please also run tests so we ensure that nothing becomes silently miscompiled?

[clackary]

@clackary Thanks! Could you please also run tests so we ensure that nothing becomes silently miscompiled?

Ah, I should have specified. My tests include both compilation and running unit tests for those projects that are configured for it. We don't have unit tests configured to run for every single project, but most of the autodiff projects are set up this way.

[kovdan01]

@asl FYI: Swift Test Linux Platform with preset looks unrelated to the changes introduced in this PR. Other build jobs are successful.