Use Clang provided public API

[ktf]

• Drop our own private copy of the headers
• Use the CMake Config

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[ktf]

@davidrohr as discussed in the meeting. This seems to compile fine.
@sawenzel any objections?

[davidrohr]

Looks good to me. However, in the past we also regularly needed to adapt the ClangTidyMain.cpp when the APIs changed. Not sure, has that stabilized now that it is a public API?

[ktf]

I do not know, I wouldn't be surprised if it was still unstable as an API. However it's not different from adapting to fair::mq / ROOT changes, no? I would tackle it as we do for other stuff, whenever clang gets updated.