[LibFuzzer] Unbreak the build on macOS which was broken by r272858.

``afl_driver.cpp`` currently relies on weak symbols which doesn't
work properly under macOS. For now fix the build by providing a
dummy implementation of ``LLVMFuzzerInitialize(...)``. This is just
a temporary measure until we fix ``afl_driver.cpp`` for macOS.

llvm-svn: 274778