[analyzer] Add method to the generic SMT API to dump the SMT formula

Summary:
New method dump the SMT formula and the Z3 implementation.

There is no test because I only used it for debugging.

However, if requested, I can add an option to the static analyzer to dump the formula (whole program? per path?), maybe something like the trimmed graph but for SMT formulas.

Reviewers: NoQ, george.karpenkov, ddcc

Reviewed By: george.karpenkov

Subscribers: xazax.hun, szepet, a.sidorin

Differential Revision: https://reviews.llvm.org/D48221

llvm-svn: 334891