[lit] Add support for attach arbitrary metrics to test results.

 - This is a work-in-progress and all details are subject to change, but I am
   trying to build up support for allowing lit to be used as a driver for
   performance tests (or other tests which might want to record information
   beyond simple PASS/FAIL).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190535 91177308-0d34-0410-b5e6-96231b3b80d8