[clang] Reapply "[cmake] Refactor clang unittest cmake" (PR #134195)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

To work on unit tests found in `clang/unittests/Sema`, I used to be able to build a target named `SemaTests`, which only required building a handful of test source files.

Do I understand correctly that, as a result of this commit, the `SemaTests` target no longer exists, and to work on / run the tests that it used to include (such as `HeuristicResolverTest.cpp), I have to build the `ClangdUnitTests` target, which requires building 1000+ test source files that I otherwise wouldn't need?

https://github.com/llvm/llvm-project/pull/134195