Add CppInterOp API dispatch mechanism

This defines the mechanism which enables dispatching of the CppInterOp API without linking to it, preventing any LLVM or Clang symbols from being leaked into the client application. Can be used to deploy CppInterOp in an environment where dynamic linking is not favourable and the only option is dlopen'ing `libClangCppInterOp.so` with `RTLD_LOCAL`

Author: aaronj0