Clang is not a complete implementation without a library, right? Whoever puts Clang together with a library becomes an integrator who provides a language implementation. It's up to that integrator to ensure that the result conforms.

Clang could be also used "freestanding". Then the user might be informed: please supply your own memcpy function with such and such characteristics, since generated code needs it.

If those characteristics are just those described in ISO C, but the compiler needs something more, then that's a problem between clang and the user.