[CMake] Support libc++ as Clang default stdlib in compiler-rt

Use libc++ when selected as default Clang stdlib and avoid checking
C++ compiler when using the in-tree version of libc++.

This is a partial alternative to D47094 that does not rely on compiler
runtime checks.

Differential Revision: https://reviews.llvm.org/D47100

llvm-svn: 333010