[Fuzzer] Avoid C++ compiler checks for fuzzer C++ library

When bulding libFuzzer as part of the toolchain, C++ library may not
have been fully built yet so the C++ compiler checks will fail, but we
don't care since we don't need C++ library anyway as we're building
our own.

Differential Revision: https://reviews.llvm.org/D45638

llvm-svn: 330075