[Property delegates] Support didSet/willSet.

A property with a delegate can provide didSet/willSet. Synthesize a setter
that calls didSet/willSet appropriately for such properties.

lib/Sema/CodeSynthesis.cpp

@@ -1807,6 +1807,12 @@ static void synthesizeSetterBody(AccessorDecl *setter,
   // Synthesize the setter for a property delegate.
   if (auto var = dyn_cast<VarDecl>(storage)) {
     if (var->hasPropertyDelegate()) {
+      if (var->getAccessor(AccessorKind::WillSet) ||
+          var->getAccessor(AccessorKind::DidSet)) {
+        synthesizeObservedSetterBody(setter, TargetImpl::Delegate, ctx);
+        return;
+      }
+
       synthesizePropertyDelegateSetterBody(setter, ctx);
       return;
     }

test/SILGen/property_delegates.swift

@@ -101,3 +101,28 @@ struct DelegateWithAccessors {
     x = y
   }
 }
+
+func consumeOldValue(_: Int) { }
+func consumeNewValue(_: Int) { }
+
+struct DelegateWithDidSetWillSet {
+  // CHECK-LABEL: sil hidden [ossa] @$s18property_delegates022DelegateWithDidSetW
+  // CHECK: function_ref @$s18property_delegates022DelegateWithDidSetWillF0V1xSivw
+  // CHECK: struct_element_addr {{%.*}} : $*DelegateWithDidSetWillSet, #DelegateWithDidSetWillSet.$x
+  // CHECK-NEXT: struct_element_addr {{%.*}} : $*Wrapper<Int>, #Wrapper.value
+  // CHECK-NEXT: assign %0 to {{%.*}} : $*Int
+  // CHECK: function_ref @$s18property_delegates022DelegateWithDidSetWillF0V1xSivW
+  var x: Int by Wrapper {
+    didSet {
+      consumeNewValue(oldValue)
+    }
+
+    willSet {
+      consumeOldValue(newValue)
+    }
+  }
+
+  mutating func test(x: Int) {
+    self.x = x
+  }
+}