[-Wunsafe-buffer-usage] Add Fixable for dereference of simple ptr arithmetic

For each expression `e` of the form `*(DRE + n)` (or `*(n + DRE)`), where
`DRE` has a pointer type and `n` is an integer literal, `e` will be
transformed to `DRE[n]` (or `n[DRE]` respectively), if
- `e` is at the left-hand side of an assignment or is an lvalue being casted to an rvalue; and
- the variable referred by `DRE` is going to be transformed to be of `std::span` type.

Reviewed by: jkorous, NoQ

Differential revision: https://reviews.llvm.org/D142795