[analyzer] Fix Z3 backend after D48205

Summary:
An assertion was added in D48205 to catch places where a `nonloc::SymbolVal` was wrapping a `loc` object.

This patch fixes that in the Z3 backend by making the `SValBuilder` object accessible from inherited instances of `SimpleConstraintManager` and calling `SVB.makeSymbolVal(foo)` instead of `nonloc::SymbolVal(foo)`.

Reviewers: NoQ, george.karpenkov

Reviewed By: NoQ

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D49430

llvm-svn: 337304