[analyzer] Moved all CSA code from the SMT API to a new header, `SMTConv.h`. NFC.

Summary:
With this patch, the SMT backend is almost completely detached from the CSA.

Unfortunate consequence is that we missed the `ConditionTruthVal` from the CSA and had to use `Optional<bool>`.

The Z3 solver implementation is still in the same file as the `Z3ConstraintManager`, in `lib/StaticAnalyzer/Core/Z3ConstraintManager.cpp` though, but except for that, the SMT API can be moved to anywhere in the codebase.

Reviewers: NoQ, george.karpenkov

Reviewed By: george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin, Szelethus

Differential Revision: https://reviews.llvm.org/D50772

llvm-svn: 340534