[cmake] Write Version.inc at cmake time, not at build time.

In SVN checkouts, clang_revision_tag is rerun on every build, even if nothing
else is dirty.  After this change, Version.inc is only written at cmake time,
so that empty builds run 0 build steps (like r191784 apparently did for git).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198704 91177308-0d34-0410-b5e6-96231b3b80d8