Move '-Wno-ambiguous-member-template' from cling to CallFunc
type in comment
Suppress warning
Implement a proper llvm::sys::path::reverse_iterator.

Since llvm::sys::path::const_iterator::operator* returns a reference to
a member of the iterator and std::reverse_iterator<T>::operator* call
T::operator* on a temporary object, llvm::sys::path:const_iterator can
not be used with std::reverse_iterator and we need to implement the reverse
iterator explicitly.

This fixes the execution of ROOT::TMetaUtils::GetFileName.
const (u)char& should only accept single character strings (helps for templates)
make sure the flags are always set to 'unsorted' after adding methods
rejig the ordering of template v.s. overloads v.s. explicit instantiations of methods
Check for NULL before assigning to TString (Coverity 53581).