[analyzer] Fix the Z3 backend always generating unsigned APSInt

Summary:
In `toAPSInt`, the Z3 backend was not checking the variable `Int`'s type and was always generating unsigned `APSInt`s.

This was found by accident when I removed:
```
    llvm::APSInt ConvertedLHS, ConvertedRHS;
    QualType LTy, RTy;
    std::tie(ConvertedLHS, LTy) = fixAPSInt(*LHS);
    std::tie(ConvertedRHS, RTy) = fixAPSInt(*RHS);
-    doIntTypePromotion<llvm::APSInt, Z3ConstraintManager::castAPSInt>(
-        ConvertedLHS, LTy, ConvertedRHS, RTy);
    return BVF.evalAPSInt(BSE->getOpcode(), ConvertedLHS, ConvertedRHS);
```

And the `BasicValueFactory` started to complain about different `signedness`.

Reviewers: george.karpenkov, NoQ, ddcc

Reviewed By: ddcc

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D49305

llvm-svn: 337169