[-Wunsafe-buffer-usage] Match unsafe pointers being casted to bool or...

[-Wunsafe-buffer-usage] Match unsafe pointers being casted to bool or participating in pointer subtractions

Add two new unique cases to the Unspecified Pointer Context (UPC),
under which we match unsafe operation patterns:
- A pointer being casted to a boolean value is in a UPC;
- A pointer participating in pointer subtraction is in a UPC.

Reviewed by: NoQ (Artem Dergachev), malavikasamak (Malavika Samak)

Differential revision: https://reviews.llvm.org/D144064