Add autodiff execution test for modify accessor differentiation

Author: asl

test/AutoDiff/validation-test/modify_accessor.swift

@@ -0,0 +1,43 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+import _Differentiation
+import StdlibUnittest
+
+var ModifyAccessorTests = TestSuite("ModifyAccessor")
+
+ModifyAccessorTests.test("SimpleModifyAccessor") {
+  struct S: Differentiable {
+    private var _x : Float
+
+    func _endMutation() {}
+
+    var x: Float {
+      get{_x}
+      set(newValue) { _x = newValue }
+      _modify {
+        defer { _endMutation() }
+        if (x > 0) {
+          yield &_x
+        } else {
+          yield &_x
+        }
+      }
+    }
+
+    init(_ x : Float) {
+      self._x = x
+    }
+  }
+
+  func modify_struct(_ x : Float) -> Float {
+    var s = S(x)
+    s.x *= s.x
+    return s.x
+  }
+
+  expectEqual((100, 20), valueWithGradient(at: 10, of: modify_struct))
+}
+
+runAllTests()
+