[Type checker] Trigger the creation of the backing storage property.

Recent improvements in the laziness of the type checker mean that we
weren't guaranteed to get the backing storage property always
synthesized for a wrapped (static) property. Make sure that we create
the backing storage property when walking a source file.

Author: DougGregor

lib/Sema/TypeCheckDecl.cpp

@@ -2413,6 +2413,9 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     (void) VD->isGetterMutating();
     (void) VD->isSetterMutating();
 
+    // Retrieve the backing property of a wrapped property.
+    (void) VD->getPropertyWrapperBackingProperty();
+
     // Set up accessors, also lowering lazy and @NSManaged properties.
     maybeAddAccessorsToStorage(VD);