Adds forward mode support for `apply` instruction with `inout` arguments.
Example of supported code:
```
func add(_ x: inout Float, _ y: inout Float) -> Float {
var result = x
result += y
return result
}
print(differential(at: 1, 1, in: add)(1, 1)) // prints "2"
```
LLVM doesn't have a stable ABI for Float16 on x86 yet; we're working with Intel to get that fixed, but we don't want to make the type available on macOS until a stable ABI is actually available, because we'd break binaries compiled before any calling convention changes if we do.
`Differentiable` conformance derivation now supports
`Differentiable.zeroTangentVectorInitializer`.
There are two potential cases:
1. Memberwise derivation: done when `TangentVector` can be initialized memberwise.
2. `{ TangentVector.zero }` derivation: done as a fallback.
`zeroTangentVectorInitializer` is a closure that produces a zero tangent vector,
capturing minimal necessary information from `self`.
It is an instance property, unlike the static property `AdditiveArithmetic.zero`,
and should be used by the differentiation transform for correctness.
Remove `Differentiable.zeroTangentVectorInitializer` dummy default implementation.
Update stdlib `Differentiable` conformances and tests.
Clean up DerivedConformanceDifferentiable.cpp cruft.
Resolves TF-1007.
Progress towards TF-1008: differentiation correctness for projection operations.
