[AutoDiff] make diff_wit_fns instead of fn_refs (#28451)
# Main change
The differentiation transform now looks up differentiability witnesses and
creates `differentiable_witness_function` instructions (derivative function
references) instead of looking up `[differentiable]` attributes and creating
`function_ref` instructions.
Differentiability witnesses are now used end-to-end, from SILGen to IRGen.
- Runtime performance impact: none with `differentiability_witness_function`
devirtualization, enabled with `-O`. (TF-994)
- Compile-time performance impact: slight regressions, to be investigated.
(TF-1013)
# Related changes
Some fixes were required to make the main change work properly.
- `LinkEntity::SecondaryPointer` is now set to `nullptr` for the
`DifferentiabilityWitness` case. Otherwise, the default wild pointer value
messes up equality comparisons.
- Handle `differentiable_witness_function` rewriting in LoadableByAddress.
- This requires support for `differentiable_witness_function` with explicit
lowered type, similar to `differentiable_function_extract`.
- Do not canonicalize differentiability witness generic signatures. Otherwise,
`τ` generic parameter types are printed in diagnostics, breaking some tests.
- As a matter of principle, canonicalization is also unnecessary; only users
that require canonical signatures should canonicalize.
- Temporarily disable test/AutoDiff/differentiable_sil_attr_roundtrip.swift.
- Round-trip SIL tests for external `@differentiable` functions are broken.
- TF-988 tracks fixing this issue.
- Move derivative lookup logic to SILOptimizer/Utils/DerivativeLookup.{h,cpp}.
- Organizing code in separate files is incremental compilation and hiding
private helper functions.
- TF-993 tracks further organization of the Differentiation.cpp mega-file.
# Next steps
TF-866 is the master issue for retroactive derivative registration.
- Remove `SILDifferentiableAttr`. (TF-898)
- `SILDifferentiableAttr` is still used in the differentiation transform for:
- Initial triggering of differentiation.
- Middle-man for differentiability witness anonicalization.
- `SILDifferentiableAttr` can be removed after these uses are rewritten.
- Lift same-file-only derivative registration limitation. (TF-1021)
- Simplify `@derivative` attribute type-checking and SILGen. (TF-835)
- SILGen `@derivative` attributes directly to differentiability witnesses.
