[llvm] r309310 - Un-revert "Teach the CMake build system to run lit's test suite. These can be run"

[Brian Gesiak via llvm-commits]

I've looked into this some more. The problem boils down to the fact
that Visual Studio is capable of specifying its build configuration at
*build* time, not CMake *configure* time. As a result, the lit.cfg
needs to be modified to use a CMake generator expression that can pull
in the current configuration VS is using to build. [1] Tricky stuff!

I have a diff that solves the problem, I'll upload it to Phabricator
in a few minutes.

[1] https://stackoverflow.com/a/39158488/679254

- Brian