Much like we silence warnings about -flto in many cases to facilitate

simplicity in build systems, silence '-stdlib=libc++' when linking. Even
if we're not linking C++ code per-se, we may be passing this flag so
that when we are linking C++ code we pick up the desired standard
library. While most build systems already provide separate C and C++
compile flags, many conflate link flags. Sadly, CMake is among them
causing this warning in a libc++ selfhost.

llvm-svn: 231559