Tweak clang testing.

 - Move CMake to using the new test runner.

 - Switch Makefiles to use the lit.site.cfg.in template.

 - Remove explicit --path arguments, instead this gets written into the site
   configuration. This means running lit from the command line should use the
   exact same configuration as is used in 'make test', assuming it can find the
   site configuration file. You still need to run 'make test' (or the cmake
   build target equivalent) at least once.

llvm-svn: 82160