[-Wunsafe-buffer-usage] Add an unsafe gadget for pointer-arithmetic operations

For -Wunsafe-buffer-usage diagnostics, we want to warn about pointer
arithmetics since resulting pointers can be used to access buffers.
Therefore, I add an `UnsafeGadget` representing general pointer
arithmetic operations.

Reviewed by: NoQ
Differential revision: https://reviews.llvm.org/D139233