Fix gcc warnings on main that cause build errors.

There are two issues here related to recent changes:
1) Our extern declarations of common RepeatedField types were removed, allowing RepeatedField::Reserve to get inlined.  This results in an `array-bounds` warning from gcc due to our memcpy call.  We had an explicit comment that this method shouldn't be inlined, and that silences the warning.

2) Using std::inserter with flat_hash_set::end() triggers a `maybe-uninitialized` warning from gcc.  This is likely an Abseil issue, and showed up recently as part of our effort to migrate to the more efficient Abseil containers.  Alternatively inserting into flat_hash_set::begin() works just fine and avoids this issue.

PiperOrigin-RevId: 501301957