[autotools->cmake] Added CMake support for the preprocessor variable CLANG_REPOSITORY_STRING.

This is a patch in a longer series of patches to add
capabilities/definitions that are present in the Autotools build chain
and not in the CMake build chain to the CMake build chain.

*NOTE* I am hacking on this in my free time. I will annotate each commit
with the [autotools->cmake] header.

llvm-svn: 188478