Enable recursing into the compiler-rt projcet with the CMake build.

This only fires if using a recent enough CMake -- compiler-rt uses a few
of the more advanced features that not everyone needs.

Please let me know if anyone sees issues with this. I'll be updating
documentation and other stuff to tell people about this.

Many thanks to Alexey for doing a ton of work to get ASan's CMake build
into a really fantastic shape. =]

llvm-svn: 162815