Add specific dependency handling for clang

[dcbaker]

Clang is fun. It has a C API library and C++ API libraries. It has the following configurations that need to be accounted for:

• libclang (The C API) can be built as either static or shared, it is the only way to get the C API
• for C++ you can build clang-cpp, which is always built as shared
• for c++ you can also get individual libs, which can be built as shared or static, but AFAICT the shared individual libs are not a supported configuration (same as LLVM).

On top of that, to get the Clang Version for the C API you need to resort to some trickery. I've checked the implementation I came up with for clang 6-17 and it works for all of those. For the C++ API it's easy, because the header required is a C++ header.

TODO:

• [x] Needs tests for the C++ API
• [ ] Needs to have CI skip expectations added

[eli-schwartz]

/cc @thesamesam

[dcbaker]

Now for the fun of getting the CI green.