[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

lib/Fuzzer/test/AFLDriverTest.cpp

@@ -11,6 +11,12 @@ extern "C" int __afl_persistent_loop(unsigned int) {
   return 0;
 }
 
+// This declaration exists to prevent the Darwin linker
+// from complaining about this being a missing weak symbol.
+extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv) {
+  return 0;
+}
+
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   return 0;
 }