[Fuzzer] Set an explicit libc++ dependency when needed

On targets that don't link internal libc++ (Fuchsia and Linux) but
use libc++ as their C++ library and libFuzzer is being built using
the just built compiler together with libc++ as part of runtimes, we
need an explicit dependency from libFuzzer object library to libc++ to
make sure the headers are available by the time we start building
libFuzzer.

Differential Revision: https://reviews.llvm.org/D48261

llvm-svn: 334928