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

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@331003 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt