[analyzer] Fixed method to get APSInt model

Summary:
This patch replaces the current method of getting an `APSInt` from Z3's model by calling generic API method `getBitvector` instead of `Z3_get_numeral_uint64`.

By calling `getBitvector`, there's no need to handle bitvectors with bit width == 128 separately.

And, as a bonus, clang now compiles correctly with Z3 4.7.1.

Reviewers: NoQ, george.karpenkov

Reviewed By: george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D49818

llvm-svn: 338020