Why are there two versions of the same functions in FlexArray, such as const type& at() const and type& at().

std::vector::at() does the same thing, so there has to be a reason, but we can't figure out what @jcmcdonald was thinking.

The odd code originated from D255, fixed with T1267. We just fixed T1410 with D362, where we found the odd code.

Docs showing the same convention with https://en.cppreference.com/w/cpp/container/vector/at