[NFC] [AAPointerInfo] OffsetAndSize is no longer an std::pair

The struct OffsetAndSize is a simple tuple of two int64_t. Treating it as a
derived class of std::pair has no special benefit, but it makes the code
verbose since we need get/set functions that avoid using "first" and "second" in
client code. Eliminating the std::pair makes this more readable.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D136745