Instead of writing every character directly into the container by using
a `back_insert_iterator` the data is buffered in an `array`. This buffer
is then inserted to the container by calling its `insert` member function.
Since there's no guarantee every container's `insert` behaves properly
containers need to opt-in to this behaviour. The appropriate standard
containers opt-in to this behaviour.
This change improves the performance of the format functions that use a
`back_insert_iterator`.
Depends on D110495
Reviewed By: ldionne, vitaut, #libc
Differential Revision: https://reviews.llvm.org/D110497
Mark de Wever