[clang-tidy] Fix a regression of readability-container-size-empty after the...

[clang-tidy] Fix a regression of readability-container-size-empty after the AST ElaboratedType change.

With 15f3cd6b, we no longer emit a
diagnostic on a real std::vector case where the size method returns a
sugar `size_type`. This patch fixes it.

```
std::vector<int> v;
if (v.size() == 0) // => no check diagnostics
  ;
```

Differential Revision: https://reviews.llvm.org/D131390