Driver: Stop linking to C++ when using sanitizers on Darwin

Sanitizers on Darwin are built as dynamic libraries, not static libraries.
Sanitizers will have their C++ dependency satisfied internally (LC_LOAD_DYLIB)
in the libclang_rt dylib. As long as the sanitizers stay dynamic and not static,
linking against C++ when enabling a sanitizer becomes over linkage.

Patch by Dave Lee!

llvm-svn: 274797