Adds the transient modifier to the cache field. This tells serialization

to avoid serializing such fields. Not having this causes serialization
to be nondeterministic, depending on whether or not the memoized accessor has
been called.

This is more robust than depending on the @lazyinit annotation, which the processor sometimes can't find.

RELNOTES=none

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=220301585

value/src/main/java/com/google/auto/value/extension/memoized/processor/MemoizeExtension.java

@@ -31,6 +31,7 @@
 import static javax.lang.model.element.Modifier.PRIVATE;
 import static javax.lang.model.element.Modifier.PUBLIC;
 import static javax.lang.model.element.Modifier.STATIC;
+import static javax.lang.model.element.Modifier.TRANSIENT;
 import static javax.lang.model.element.Modifier.VOLATILE;
 import static javax.lang.model.type.TypeKind.VOID;
 import static javax.lang.model.util.ElementFilter.methodsIn;
@@ -324,11 +325,11 @@ private boolean pullDownMethodAnnotation(AnnotationMirror annotation) {
 
       /**
        * Builds a {@link FieldSpec} for use in property caching. Field will be {@code private
-       * volatile} and have the given type and name. If the @LazyInit annotation is available it is
-       * added as well.
+       * transient volatile} and have the given type and name. If the @LazyInit annotation is
+       * available it is added as well.
        */
       private FieldSpec buildCacheField(TypeName type, String name) {
-        FieldSpec.Builder builder = FieldSpec.builder(type, name, PRIVATE, VOLATILE);
+        FieldSpec.Builder builder = FieldSpec.builder(type, name, PRIVATE, TRANSIENT, VOLATILE);
         if (lazyInitAnnotation.isPresent()) {
           builder.addAnnotation(lazyInitAnnotation.get());
           builder.addAnnotation(SUPPRESS_WARNINGS);