Add support for const a prefix when dropping std::


git-svn-id: https://root.cern.ch/svn/root/trunk@48743 27541ba8-7e3a-0410-8455-c3a389f83636
In the case of a type(def) that was declared in a
class template but is *not* type dependent.  In clang, it gets
attached to the class template declaration rather than any
specific class template instantiation.   This result in 'odd'
fully qualified typename:
   vector<_Tp,_Alloc>::size_type
Make the situation is 'useable' but looking a bit odd by picking a
random instance as the declaring context.


git-svn-id: https://root.cern.ch/svn/root/trunk@48744 27541ba8-7e3a-0410-8455-c3a389f83636