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

[Tom Honermann via cfe-commits]

tahonermann wrote:

I ended up spending a lot of time barking down the wrong rabbit hole due to these changes. @rnk, please see https://github.com/llvm/llvm-project/issues/140799.

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