[AutoDiff] Re-enable LoadableByAddress. (#27923)
The LoadableByAddress transform rewrites function types. This caused
verification failures in lowered SIL for `differentiable_function` and
`differentiable_function_extract` instructions, which have precise type
verification rules.
Now, verification has been relaxed for these instructions in lowered SIL.
- `differentiable_function_extract` can now have an explicit extractee type
in lowered SIL: parsing/printing/serialization support have been added.
- LoadableByAddress rewrites `differentiable_function_extract` instructions
using explicit address-lowered function type, similar to other function
conversion instructions.
- Add SIL FileCheck and runtime tests.
