The SIL type lowering logic for AutoDiff gets the substituted generic signature
mixed up with the invocation generic signature, so it tries to ask questions
about DependentMemberTypes in a signature with no requirements. This triggers
assertions when the requirement machine is enabled.
Disable the requirement machine until this is fixed.
Splits the large `forward_mode.swift` test file into 3 files:
- `forward_mode_simple.swift`
- `forward_mode_array.swift`
- `forward_mode_simd.swift`
This significantly speeds up testing time when tests are run in parallel.