[-Wunsafe-buffer-usage] Fixits for assignments to array subscript expressions

Let generate fix-its to make assignments' left-hand side of the form
`dre[e]` safe if `e` is known to be non-negative.

Commit on behalf of jkorous (Jan Korous)

Reviewed by: NoQ (Artem Dergachev)

Differential revision: https://reviews.llvm.org/D142794