DenseSet: Appease msc18 to define derived constructors explicitly.

msc18 doesn't recognize "using BaseT::BaseT;"

  llvm\include\llvm/ADT/DenseSet.h(213) : error C2875: using-declaration causes a multiple declaration of 'BaseT'
          llvm\include\llvm/ADT/DenseSet.h(214) : see reference to class template instantiation 'llvm::DenseSet<ValueT,ValueInfoT>' being compiled
  llvm\include\llvm/ADT/DenseSet.h(231) : error C2875: using-declaration causes a multiple declaration of 'BaseT'
          llvm\include\llvm/ADT/DenseSet.h(232) : see reference to class template instantiation 'llvm::SmallDenseSet<ValueT,InlineBuckets,ValueInfoT>' being compiled

llvm-svn: 284570