[AutoDiff] Support derivative registration for more declaration kinds.

[dan-zheng]

Make @differentiating and @transposing attributes support more original
declaration kinds: computed properties, subscripts, and initializers.

• Change DifferentiatingAttr and TransposingAttr to store an
  AbstractFunctionDecl representing the original declaration, instead of a
  FuncDecl.
• Change attribute parsing to support initializer/subscript DeclNames.
• Make TypeChecker::lookupFuncDecl a static function in TypeCheckAttr.cpp.
• Assorted parsing and type-checking gardening.

@differentiating now has feature parity with
@differentiable(jvp: ..., vjp: ...) for derivative registration.

This is a necessary step towards making @differentiating and @transposing
the canonical mechanism for registering derivative/transpose functions.

Registering non-func declaration derivatives with @differentiable attribute
jvp:/vjp: labels is now explicitly rejected.

Resolves TF-281.

Todos:

• Upstream changes to previously upstreamed code.
• TF-997: support @transposing attribute with initializer original
  declarations.
• TF-988: do not reuse @differentiable attribute type-checking diagnostics
  for @differentiating/@transposing attribute type-checking.

---

Newly supported:

struct Foo: Differentiable {}

// Computed properties.
extension Foo {
  var computedProperty: Float { 1 }

  @differentiating(computedProperty)
  func vjpComputedProperty() -> (value: Float, pullback: (Float) -> TangentVector) { ... }
}

// Initializers.
extension Foo {
  init(_ x: Float) {}

  @differentiating(init, wrt: x)
  static func vjpInit(_ x: Float) -> (value: Foo, pullback: (TangentVector) -> Float) { ... }
}

// Subscripts.
extension Foo {
  subscript() -> Float { 1 }

  @differentiating(subscript)
  func vjpSubscript() -> (value: Float, pullback: (Float) -> TangentVector) { ... }
}

Newly diagnosed:

struct Bar: Differentiable {
  @differentiable(vjp: computedPropertyVJP)
  func instanceMethod() -> Float { 1 }

  var computedPropertyVJP: (Float, (Float) -> TangentVector) { ... }
}

// error: registered derivative 'computedPropertyVJP' must be a 'func' declaration
//   @differentiable(vjp: computedPropertyVJP)
//                        ^

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

Happy to address feedback later!