[analyzer] ApiModeling: Add buffer size arg constraint with multiplier involved

Summary:
Further develop the buffer size argumentum constraint so it can handle sizes
that we can get by multiplying two variables.

Reviewers: Szelethus, NoQ, steakhal

Subscribers: whisperity, xazax.hun, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, donat.nagy, dkrupp, gamesh411, Charusso, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77148