[AutoDiff] Make @noDerivative attribute imply non-varying semantics.

[dan-zheng]

Previously, @noDerivative could only be declared on stored properties in
Differentiable-conforming structs and classes.

Now, @noDerivative can be declared on all function-like declarations:
func, var, init, and subscript.

@noDerivative attribute now also implies @_semantics("autodiff.nonvarying").

SIL values produced from @noDerivative declarations will never be marked as
varying by differentiable activity analysis. These values do not need a
derivative.

Marking declarations as @noDerivative is a usability improvement over using
withoutDerivative(at:) at use sites. However, @noDerivative is invasive
because it requires annotation on original declarations. There may exist a more
elegant solution.

---

Example:

@differentiable
func sum(_ array: [Float]) -> Float {
  var result: Float = 0
  for i in array.indices {
    result = result + array[i]
  }
  return result
}

Array.indices calls the default protocol implementation RandomAccessCollection.indices, which is defined in the stdlib and currently has no differentiation-related annotations.

Thus, the code above triggers a non-differentiability error:

$ swift test.swift
test.swift:1:2: error: function is not differentiable
@differentiable
~^~~~~~~~~~~~~~
test.swift:2:6: note: when differentiating this function definition
func sum(_ array: [Float]) -> Float {
     ^
test.swift:4:18: note: cannot differentiate through a non-differentiable result; do you want to use 'withoutDerivative(at:)'?
  for i in array.indices {
                 ^

---

Old workaround: use withoutDerivative(at: array.indices. withoutDerivative(at:) has non-varying semantics.

@differentiable
func sum(_ array: [Float]) -> Float {
  var result: Float = 0
  for i in withoutDerivative(at: array.indices) {
    result = result + array[i]
  }
  return result
}
// No non-differentiability error.

However, applying withoutDerivative(at:) is cumbersome for properties/functions like Array.indices or Array.count, which are always expected to have non-varying semantics.

Example of excessive withoutDerivative(at:) usage: poor usability.

---

New workaround: annotate RandomAccessCollection.indices with @noDerivative. This is an alternative to applying withoutDerivative(at:), achieving the same effect.

@differentiable
func sum(_ array: [Float]) -> Float {
  var result: Float = 0
  for i in array.indices {
    result = result + array[i]
  }
  return result
}
// No non-differentiability error for original code.

Note that this patch does not actually add @noDerivative to RandomAccessCollection.indices in the stdlib. But adding @noDerivative to RandomAccessCollection.indices is verified to work.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

@swift-ci Please test tensorflow