At the request of Michael Spencer, make the VCS version detection logic

At the request of Michael Spencer, make the VCS version detection logic
in CMake a bit more handy. Previously we would get such charming
versions as the following for revision NNNN and commit-ish XXXXX:
  3.1svnsvn-rNNNN
  3.1svngit-svn-rNNNN
  3.1svngit-svn-XXXXX

The mechanism selecting betwene the latter two was particularly odd, and
didn't work with all of the ways git-svn repos are set up apparently. It
also misses an important point -- both the revision *and* the git commit
might be relevant when working on a local branch some distance from
mainline. The new logic does several things:

1) It strips the redundant initial 'svn'.
2) It always looks for a git-svn revision number base, and when found
   includes it in the version.
3) If the git commit-ish for the current HEAD is not exactly that
   revision number, it is also included.

The resulting strings should roughly be:
  3.1svn-rNNNN
  3.1git-svn-rNNNN
  3.1git-svn-rNNNN-XXXXX

Suggestions on formatting etc always welcome. =] I've only looked at the
LLVM version string here, not Clang's (yet).

Note that the commit-ish reported is *not* terribly accurate. It updates
when 'cmake' is run, not when the binary is built. Still, it may be
better than nothing, especially if people have fairly long-lived git
repos and branches. This is not a new limitation, just didn't want
anyone to be surprised.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146323 91177308-0d34-0410-b5e6-96231b3b80d8