[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/Constants.java

@@ -0,0 +1,8 @@
+package pkg;
+
+import lombok.experimental.FieldNameConstants;
+
+@FieldNameConstants
+public class Constants {
+	String test;
+}