[clang-tidy] Omit cases where loop variable is not used in loop body in

performance-for-range-copy check.

Summary:
The upstream change r336737 make the check too smart to fix the case
where loop variable could be used as `const auto&`.

But for the case below, changing to `const auto _` will introduce
an unused complier warning.

```
for (auto _ : state) {
  // no references for _.
}
```

This patch omit this case, and it is safe to do it as the case is very rare.

Reviewers: ilya-biryukov, alexfh

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D50447

llvm-svn: 339415