Fix TensorFlow module compilation.

[dan-zheng]

Fix KeyPathIterable derived conformances: KeyPathIterable.allKeyPaths
synthesis logic now requires different key-path expression coercion logic
to accommodate upstream type-checker changes.

---

Summary:

// Generated code for `KeyPathIterable.allKeyPaths`.
var allKeyPaths: [PartialKeyPath<Self>] {
  // Ideal code: TF-575.
  // error: cannot convert parent type 'main.TF575_LayerNorm<Scalar>' to expected type 'main.TF575_LayerNorm<Scalar>'
  [\Self.offset, \Self.scale]

  // Previously working code before swift-DEVELOPMENT-SNAPSHOT-2019-10-24-a.
  // error: cannot convert value of type '[WritableKeyPath<TF575_LayerNorm<Scalar>.TangentVector, Tensor<Scalar>>]' to type 'Array<Element>' in coercion
  // [\Self.offset, \Self.scale] as [PartialKeyPath<Self>]

  // Newly generated code.
  // [\Self.offset as PartialKeyPath<Self>, \Self.scale as PartialKeyPath<Self>]
}

---

See TF-575 for more information.
I'll minimize the type-checker issues and file upstream bugs.

[pschuh]

Test?

[dan-zheng]

Test?

Done in 9efd87e.

[dan-zheng]

@swift-ci Please clean test tensorflow

[dan-zheng]

TensorFlow module now compiles.

Remaining test failures:

Failing Tests (10):
    Swift(linux-x86_64) :: SILOptimizer/invalid_escaping_captures.swift
    Swift(linux-x86_64) :: SILOptimizer/infinite_recursion.swift
    Swift(linux-x86_64) :: SILOptimizer/exclusivity_static_diagnostics.swift
    Swift(linux-x86_64) :: SILOptimizer/generalized_accessors.swift
    Swift(linux-x86_64) :: SILOptimizer/noescape_param_exclusivity.swift
    Swift(linux-x86_64) :: SILOptimizer/diagnostic_constant_propagation_int_arch64.swift
    Swift(linux-x86_64) :: SILOptimizer/definite_init_failable_initializers_diagnostics.swift
    Swift(linux-x86_64) :: SILOptimizer/definite_init_diagnostics.swift
    Swift(linux-x86_64) :: SILOptimizer/mandatory_inlining.swift
    Swift(linux-x86_64) :: SILOptimizer/constant_propagation_diagnostics.swift