Got rid of the `Z3ConstraintManager` class

Now, instead of passing the reference to a shared_ptr, we pass the shared_ptr instead.

I've also removed the check if Z3 is present in CreateZ3ConstraintManager as this function already calls CreateZ3Solver that performs the exactly same check.

Differential Revision: https://reviews.llvm.org/D54976

llvm-svn: 353371