[clang] [clang] Build the Z3 mock module via CMake (PR #146284)

[Michał Górny via cfe-commits]

mgorny wrote:

Hmm, but I see it's already part of #145895 — and I think that PR will apply cleanly once this change is made, so I think it's better to keep them separately and focused on a single aspect.

https://github.com/llvm/llvm-project/pull/146284