Summary: Implement new Z3 constraint manager backend.

Reviewers: zaks.anna, dcoughlin, NoQ, xazax.hun

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D28952

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299463 91177308-0d34-0410-b5e6-96231b3b80d8