please add clang support

[bsbandwl]

Please add clang/LLVM support.

[Letme]

It already works, do you want target file examples or something?

[g-berthiaume]

The relevant documentation: https://github.com/ThrowTheSwitch/Ceedling/blob/master/docs/CeedlingPacket.md#tool-element-runtime-substitution-notational-substitution

That said, I would appreciate a clang example in the docs. It seems like this would be a pretty common use case.