LLVM's CMake is now using a feature that wasn't preasant in CMake 2.8.7,

so bump the minimum version in the standalone Clang CMake project as
well.

As I mentioned on the LLVM commit version of this, if this causes any
trouble for folks, just let me know. I'm trying to avoid re-implementing
functionality in CMake, but I will if there are problems using the newer
versions.

llvm-svn: 199152