Replace fuzzer::FuzzerDriver's INTERFACE marking with new LLVMRunFuzzerDriver.

This adds a new extern "C" function that serves the same purpose. This removes the need for external users to depend on internal headers in order to use this feature. It also standardizes the interface in a way that other fuzzing engines will be able to match.

Patch By: IanPudney

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D84561