CMake: only try to find the Z3 package when `CLANG_ANALYZER_BUILD_Z3` is ON

This avoids the spurious message:

 Could NOT find Z3 (missing:  Z3_LIBRARIES Z3_INCLUDE_DIR) (Required is at least version "4.5")

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311858 91177308-0d34-0410-b5e6-96231b3b80d8