Improve compatibility with certain STL's
We use customized libc++ which uses raw pointers as std::vector::iterators.
As per [expr.pre.incr](https://eel.is/c++draft/expr.compound#expr.pre.incr), builtin `operator++` can only be applied to lvalue, while `std::vector::begin()` returns an rvalue.
See [this](https://godbolt.org/z/d3a1aKTWP) godbolt snippet for the details.
