[AutoDiff] Add SR-12642 negative test.

Add SR-12642 negative test: crash regarding `Differentiable` derived
conformances and redeclared properties.

Author: dan-zheng

test/AutoDiff/compiler_crashers/sr12642-differentiable-derivation-redeclared-property.swift

@@ -0,0 +1,18 @@
+// RUN: not --crash %target-swift-frontend -typecheck %s
+
+// SR-12642: Crash regarding `Differentiable` derived conformances and redeclared properties.
+
+import _Differentiation
+
+@propertyWrapper
+struct Wrapper<Value> {
+  var wrappedValue: Value
+}
+
+struct Generic<T> {}
+extension Generic: Differentiable where T: Differentiable {}
+
+struct WrappedProperties: Differentiable {
+  @Wrapper var int: Generic<Int>
+  @Wrapper var int: Generic<Int>
+}