[builtin_object_size] Basic support for posix_memalign

It actually implements support for seeing through loads, using alias analysis to
refine the result.

This is rather limited, but I didn't want to rely on more than available
analysis at that point (to be gentle with compilation time), and it does seem to
catch common scenario, as showcased by the included tests.

Differential Revision: https://reviews.llvm.org/D122431