[AutoDiff] Add SR-12656 negative test. (swiftlang#31673)

Add negative test for SR-12656: differentiation transform crash for opaque
result types.

Author: dan-zheng

test/AutoDiff/compiler_crashers/sr12656-differentiation-opaque-result-type.swift

@@ -0,0 +1,28 @@
+// RUN: not --crash %target-swift-frontend -disable-availability-checking -emit-sil -verify %s
+// REQUIRES: asserts
+
+// SR-12656: Differentiation transform crashes for original function with opaque
+// result type.
+//
+// If supporting this is non-trivial, we could reject earlier during Sema.
+
+import _Differentiation
+
+@differentiable
+func opaqueResult(_ x: Float) -> some Differentiable { x }
+
+// swift: swift/lib/SILOptimizer/Differentiation/PullbackEmitter.cpp:244: void swift::autodiff::PullbackEmitter::addAdjointValue(swift::SILBasicBlock *, swift::SILValue, swift::autodiff::AdjointValue, swift::SILLocation): Assertion `newAdjointValue.getType() == getRemappedTangentType(originalValue->getType())' failed.
+// Stack dump:
+// ...
+// 1.      Swift version 5.3-dev (LLVM f66b332548, Swift 05c47da664)
+// 2.      While evaluating request ExecuteSILPipelineRequest(Run pipelines { Guaranteed Passes } on SIL for main.main)
+// 3.      While running pass #15 SILModuleTransform "Differentiation".
+// 4.      While processing // differentiability witness for opaqueResult(_:)
+// sil_differentiability_witness hidden [parameters 0] [results 0] @$s4main12opaqueResultyQrSfF : $@convention(thin) (Float) -> @out @_opaqueReturnTypeOf("$s4main12opaqueResultyQrSfF", 0) 🦸 {
+// }
+//  on SIL function "@$s4main12opaqueResultyQrSfF".
+//  for 'opaqueResult(_:)' (at swift/test/AutoDiff/compiler_crashers/sr12656-differentiation-opaque-result-type.swift:13:1)
+// 5.      While generating VJP for SIL function "@$s4main12opaqueResultyQrSfF".
+//  for 'opaqueResult(_:)' (at swift/test/AutoDiff/compiler_crashers/sr12656-differentiation-opaque-result-type.swift:13:1)
+// 6.      While generating pullback for SIL function "@$s4main12opaqueResultyQrSfF".
+//  for 'opaqueResult(_:)' (at swift/test/AutoDiff/compiler_crashers/sr12656-differentiation-opaque-result-type.swift:13:1)