[fixes #3529] Set real field initializer source

Eclipse copies the field initializer source code if it is a constant.
For generated nodes we have to read it from the ast node and not from
the source file.

test/eclipse/resource/noerrors/fieldNameConstantsInAnnotation/Usage.java

@@ -0,0 +1,5 @@
+package pkg;
+
+@Annotation(Constants.Fields.test)
+public class Usage {
+}