[CMAKE] Move version control macros to AddLLVM.cmake so they can be reused by clang, etc.

Summary:
Move version control macros, find_first_existing_file and
find_first_existing_vc_file to AddLLVM.cmake so they can be reused by sub projects
like clang.

Differential Revision: https://reviews.llvm.org/D36971

llvm-svn: 312419