[CMake] Remove CLANG_APPEND_VC_REV option

I added this option in r257827 to try and add compatibility with autoconf. At the time I misunderstood the problem.

Our CMake automatically generates the SVN revision information and generates a build action to update it so builds don't need to be re-configured on SCM update (which is a better solution than we had in autoconf).

The problem I was actually seeing was isolated cases where SVN revision information isn't available because the repository structures have been removed. This happens in some automated testing systems.

This patch allows SVN_REVISION to be overridden if the build configuration could not find the SCM repository structures, and removes the code from my original patch because it is unnecessary.

llvm-svn: 273714