[c++1z] Implement N3994: a range-based for loop can declare a variable with super-terse notation

  for (x : range) { ... }

which is equivalent to

  for (auto &&x : range) { ... }


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211267 91177308-0d34-0410-b5e6-96231b3b80d8