Fold DOMElementNSImpl::getBaseURI and DOMElementImpl::getBaseURI into DOMElementImpl, so that it still works when an element contains both DOMAttrNSImpl and DOMAttrImpl attributes

git-svn-id: https://svn.apache.org/repos/asf/xerces/c/trunk@529271 13f79535-47bb-0310-9956-ffa450edef68