Update C++ status pages for Clang 4 branch:

 * Update version number in DR tests from 4.0 to 4
 * Teach make_cxx_dr_status script about version numbers that don't contain a
   period.
 * Update cxx_status.html and cxx_dr_status.html to list Clang 4 features as
   "Clang 4" rather than "SVN"

Clang 4 features are still listed in yellow rather than green until release.

llvm-svn: 291871