[clang-tools-extra] [clang-doc] create a separate cmake file for clang-doc's lit tests (PR #165935)

[Erick Velez via cfe-commits]

evelez7 wrote:

> If we're doing that, should we also consider having our own unittests target?

Yes, we should. I want to do that next. The thing is that the `add_lit_testsuite` CMake macro thing exposes a target in `build.ninja`. AFAIK, no project in clang-tools-extra has a special command (like `check-clang-extra-unit-clangd`). So I'll have to do some CMake digging. 🙃

https://github.com/llvm/llvm-project/pull/165935