Fix loop-convert for const references to containers.

Previously we would use a non-const loop variable in the range-based
loop for:
void f(const std::vector<int> &v) {
  for (size_t i = 0; i < v.size(); ++i) {
Now we use const auto&.

Note that we'll also want to use a copy at least for simple types.

llvm-svn: 248418