[-Wunsafe-buffer-usage] Add warnings for unsafe buffer accesses by array subscript operations

Add an unsafe Gadget in the Unsafe Buffer Usage anslysis to represent
(and search (via matchers) for) unsafe buffer-accessing array subscript operations.

Reviewed By: NoQ, jkorous, aaron.ballman, xazax.hun

Differential Revision: https://reviews.llvm.org/D137379