[-Wunsafe-buffer-usage] Introduce std::array fixits (#80084)

Array subscript on a const size array is not bounds-checked. The idiomatic
replacement is std::array which is bounds-safe in hardened mode of libc++.

This commit extends the fixit-producing machine to consider std::array as a
transformation target type and teaches it to handle the array subscript on const
size arrays with a trivial (empty) fixit.