[libFuzzer] Support using libc++

This is needed in case the users of libFuzzer use libc++ in their
code, which the fuzz target (libFuzzer) will be linked against.
When libc++ source is available, we build a private version of it
and link it against libFuzzer which allows using the same static
library against codebases which use both libc++ and libstdc++.

Differential Revision: https://reviews.llvm.org/D37631

llvm-svn: 322604