[CMake] Make simple source used for CMake checks a C file

The source being compiled is plain C, but using .cc extension forces it
to be compiled as C++ which requires a working C++ compiler including
C++ library which may not be the case when we're building compiler-rt
together with libcxx as part of runtimes build.

Differential Revision: https://reviews.llvm.org/D46152

llvm-svn: 331003