[clang] [cmake] Refactor clang unittest cmake (PR #133545)

[Petr Hosek via cfe-commits]

https://github.com/petrhosek approved this pull request.

This is definitely an improvement over status quo! Ideally, developers wouldn't have to list Clang libraries separately from other libraries but that would require some non-trivial build system refactoring that would be better done in follow up changes.

https://github.com/llvm/llvm-project/pull/133545