[CMake] Use a different source depending on C++ support

When using system C++ library, assume we have a working C++ compiler and
try to compile a complete C++ program. When using in tree C++ library,
only check the C compiler since the C++ library likely won't have been
built yet at time of running the check.

Differential Revision: https://reviews.llvm.org/D47169

llvm-svn: 332924