test/lit.cfg: Enhance getRegisteredTargets() to retrieve +Asserts.

  - s/getRegisteredTargets/get_llc_props/g
  - get_llc_props() returns dict {"set_of_targets", "enable_assertions"}
  - Enable the feature "asserts".
    test/Driver/darwin-verify-debug.c had expected it.

llvm-svn: 145231