[PATCH] Handle LLVM_APPEND_VC_REV with LLVM cmake module

LLVM uses own CMake module VersionFromVCS.cmake to lookup repository and
revision when LLVM_APPEND_VC_REV is ON, and it supports additionally getting
information from Git SVN and Git mirrors.

Clang (cfe) patches should go to the cfe-commits list, not cfe-dev.
--paulr