Stub generation malforms annotation values

This issue was introduced by https://issues.apache.org/jira/browse/GROOVY-6085 and the fix https://github.com/apache/groovy/commit/561fad780794a5ac6be81818d2f123496e6c9bad#diff-fb5c7aed8d75294b5fbca45b00da5e20968d33704cb81720503ee3601df30fecR732

That commit replaces `$` with `.` in annotation values, however it seems that should only happen if the value is a class expression. For things like string literals it is changing the string value which is causing other problems.