[SE-0258] Fix crash with implicitly-initialized backing storage.

Fixes SR-10830 / rdar://problem/51443591

Author: DougGregor

lib/AST/Decl.cpp

@@ -5478,6 +5478,11 @@ bool VarDecl::isPropertyWrapperInitializedWithInitialValue() const {
   // If there is no initializer, the initialization form depends on
   // whether the property wrapper type has an init(initialValue:).
   if (!isParentInitialized()) {
+    // If it's default-initializable, that doesn't use an initial value.
+    if (auto *PBD = getParentPatternBinding())
+      if (PBD->isDefaultInitializable())
+        return false;
+
     auto wrapperTypeInfo = getAttachedPropertyWrapperTypeInfo();
     return wrapperTypeInfo.initialValueInit != nullptr;
   }

test/SILGen/property_wrappers.swift

@@ -269,6 +269,34 @@ extension ClassUsingWrapper {
   }
 }
 
+// 
+@_propertyWrapper
+struct WrapperWithDefaultInit<T> {
+  private var storage: T?
+
+  init() {
+    self.storage = nil
+  }
+  
+  init(initialValue: T) {
+    self.storage = initialValue
+  }
+
+  var value: T {
+    get { return storage! }
+    set { storage = newValue }
+  }
+}
+
+class UseWrapperWithDefaultInit {
+  @WrapperWithDefaultInit var name: String
+}
+
+// CHECK-LABEL: sil hidden [transparent] [ossa] @$s17property_wrappers25UseWrapperWithDefaultInitC5$name33_F728088E0028E14D18C6A10CF68512E8LLAA0defG0VySSGvpfi : $@convention(thin) () -> @owned WrapperWithDefaultInit<String>
+// CHECK: function_ref @$s17property_wrappers22WrapperWithDefaultInitVACyxGycfC
+// CHECK: return {{%.*}} : $WrapperWithDefaultInit<String>
+
+
 // CHECK-LABEL: sil_vtable ClassUsingWrapper {
 // CHECK:  #ClassUsingWrapper.x!getter.1: (ClassUsingWrapper) -> () -> Int : @$s17property_wrappers17ClassUsingWrapperC1xSivg   // ClassUsingWrapper.x.getter
 // CHECK:  #ClassUsingWrapper.x!setter.1: (ClassUsingWrapper) -> (Int) -> () : @$s17property_wrappers17ClassUsingWrapperC1xSivs // ClassUsingWrapper.x.setter