[analyzer] Fixed bitvector from model always being unsigned

Summary:
Getting an `APSInt` from the model always returned an unsigned integer because of the unused parameter.

This was not breaking any test case because no code relies on the actual value of the integer returned here, but rather it is only used to check if a symbol has more than one solution in `getSymVal`.

Reviewers: NoQ, george.karpenkov

Reviewed By: george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin, Szelethus, donat.nagy, dkrupp

Differential Revision: https://reviews.llvm.org/D53637

llvm-svn: 345283