CMake: enable building the clang-format vs plugin

This makes it possible to build the clang-format vs plugin from the cmake build.
It is a hack, as it shells out to "devenv" to actually build it, but it's hidden
away in a corner behind a flag, and it provides a convenient way of building the
plug-in from the command-line together with the rest of clang.

Differential Revision: http://llvm-reviews.chandlerc.com/D2310