Suppress serialization warnings. (#2502)

Recent versions of javac emit a warning if a serializable class has
a non-transient instance field with a declared type that does not
inherit `Serializable`. In this context, we know that the actual
values will always be serializable.

gson/src/main/java/com/google/gson/internal/$Gson$Types.java

@@ -496,9 +496,16 @@ public static boolean requiresOwnerType(Type rawType) {
     return false;
   }
 
+  // Here and below we put @SuppressWarnings("serial") on fields of type `Type`. Recent Java
+  // compilers complain that the declared type is not Serializable. But in this context we go out of
+  // our way to ensure that the Type in question is either Class (which is serializable) or one of
+  // the nested Type implementations here (which are also serializable).
   private static final class ParameterizedTypeImpl implements ParameterizedType, Serializable {
+    @SuppressWarnings("serial")
     private final Type ownerType;
+    @SuppressWarnings("serial")
     private final Type rawType;
+    @SuppressWarnings("serial")
     private final Type[] typeArguments;
 
     public ParameterizedTypeImpl(Type ownerType, Type rawType, Type... typeArguments) {
@@ -564,6 +571,7 @@ private static int hashCodeOrZero(Object o) {
   }
 
   private static final class GenericArrayTypeImpl implements GenericArrayType, Serializable {
+    @SuppressWarnings("serial")
     private final Type componentType;
 
     public GenericArrayTypeImpl(Type componentType) {
@@ -598,7 +606,9 @@ public GenericArrayTypeImpl(Type componentType) {
    * is set, the upper bound must be Object.class.
    */
   private static final class WildcardTypeImpl implements WildcardType, Serializable {
+    @SuppressWarnings("serial")
     private final Type upperBound;
+    @SuppressWarnings("serial")
     private final Type lowerBound;
 
     public WildcardTypeImpl(Type[] upperBounds, Type[] lowerBounds) {