CMake: Report an error if there is an unknown .cpp file in a source

CMake: Report an error if there is an unknown .cpp file in a source
directory.

This is useful in case someone who works with the config&make build
system forgot to add a file to its CMakeLists.txt. Instead of
obtaining undefined references at link time, cmake will complain at
configure time on the first build after a svn update.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85817 91177308-0d34-0410-b5e6-96231b3b80d8