Summary:
std::vector<bool>::iterator::operator* returns a temporary proxy bool accessor
in many implementations.  So range-based for loops over it don't work if we
assume non-const lvalue-ref binding, eg.
```
for (auto& ref : vectorBools) { ... }
```
Won't compile.  Silly std::vector<bool>.

Reviewed By: SmithAndr

Differential Revision: D30833990

fbshipit-source-id: cb8d53a48eb2a40587911ad2f0c50b99a472a59c