Fix for ROOT-5513

Item https://sft.its.cern.ch/jira/browse/ROOT-5513 was fixed.
It was blocking the compilation of CMSSW.
Now xml files of the form
<lcgdict>
 <class name="edm::ErrorSummaryEntry" ClassVersion="10">
  <version ClassVersion="10" checksum="3974422774"/>
 </class>
 <class name="edm::ELseverityLevel" ClassVersion="10">
  <version ClassVersion="10" checksum="2388166397"/>
 </class>
 <class name="std::vector<edm::ErrorSummaryEntry>"/>
</lcgdict>

are properly digested by rootcling and the information passed
to TClass. Since TMapAttributeMap implements string and int
properties and they cover the 99% of the plausible usecases,
integer properties which cannot be stored in int variables
(like the checksums of CMS!) are stored as strings.