More git-svn compatible version string, by request.

If you're using git-svn, the clang and llvm repository will typically
map to a different revision.

Before we had:
clang version 3.1 (trunk 152167 trunk 152162)

After this change:
clang version 3.1 (trunk 152167) (llvm/trunk 152162)

So it's self-descriptive with an extra parens group. Which is more
compatible with version string parsers is probably debatable, but this
style was requested.

llvm-svn: 152183