Fix loop-convert for trivially copyable types.

Previously, we would rewrite:
void f(const vector<int> &v) {
  for (size_t i = 0; i < v.size(); ++i) {
to
  for (const auto &elem : v) {

Now we rewrite it to:
  for (auto elem : v) {
(and similarly for iterator based loops).

llvm-svn: 248438