[analyzer] Move canReasonAbout from Z3ConstraintManager to SMTConstraintManager

Summary:
This patch moves the last method in `Z3ConstraintManager` to `SMTConstraintManager`: `canReasonAbout()`.

The `canReasonAbout()` method checks if a given `SVal` can be encoded in SMT. I've added a new method to the SMT API to return true if a solver can encode floating-point arithmetics and it was enough to make `canReasonAbout()` solver independent.

As an annoying side-effect, `Z3ConstraintManager` is pretty empty now and only (1) creates the Z3 solver object by calling `CreateZ3Solver()` and (2) instantiates `SMTConstraintManager`. Maybe we can get rid of this class altogether in the future: a `CreateSMTConstraintManager()` method that does (1) and (2) and returns the constraint manager object?

Reviewers: george.karpenkov, NoQ

Reviewed By: george.karpenkov

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

Differential Revision: https://reviews.llvm.org/D53694

llvm-svn: 345284