[AutoDiff] Diagnose unsupported coroutine differentiation.

[dan-zheng]

Coroutines are functions that can yield values and suspend/resume execution.
SIL has dedicated coroutine types. Coroutines are applied via begin_apply.
The read and modify accessors are coroutines.

Coroutine differentiation requires extra support, so it should be diagnosed
for now.

---

Differentiation transform:

• Generalize differentiation utilities from ApplyInst to FullApplySite.
  • bool isArrayLiteralIntrinsic(FullApplySite)
  • void forEachApplyDirectResult(FullApplySite, llvm::function_ref<...>)
    • apply: direct result is the apply result, unless the apply result
      has tuple-type and a single destructure_tuple user. Then, direct results
      are the destructure_tuple results.
    • begin_apply: direct results are the begin_apply results.
    • try_apply: direct results are successor blocks' arguments.
  • bool LinearMapInfo::shouldDifferentiateApplySite(FullApplySite)
• Generalize activity analysis from ApplyInst to FullApplySite.
  • Propagate variedness for FullApplySite through direct/indirect results and
    inout arguments.
  • Propagate usefulness for FullApplySite through arguments.
• Diagnose begin_apply instructions with active arguments and results in
  PullbackEmitter::visitBeginApplyInst.

Sema:

• Diagnose @differentiable attribute on read and modify coroutines.

Resolves TF-1081.

TF-1080 tracks coroutine differentiation support.
TF-1083 tracks throwing function differentiation support.

---

Example:

@differentiable
func foo(array: [Float], x: Float) -> Float {
  var array = array
  // `read` and `modify` accessors are coroutines.
  // Array subscript assignment below calls `Array.subscript.modify`.
  array[0] = x * x
  return array[0]
}
print(valueWithGradient(at: [3, 4], 5, in: foo))

// Before: silently incorrect results.
// (value: 25.0, gradient: ([1.0, 0.0], 0.0)).

// After: diagnosed.
// coroutine.swift:1:2: error: function is not differentiable
// @differentiable
// ~^~~~~~~~~~~~~~
// coroutine.swift:2:6: note: when differentiating this function definition
// func foo(array: [Float], x: Float) -> Float {
//   ^
// coroutine.swift:5:12: note: differentiation of coroutine calls is not yet supported
//   array[0] = x * x
//     ^

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

Merging now. Happy to address review feedback later!