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

Summary:
Depends on https://reviews.llvm.org/D35879.

This reverts rL257268, which in turn was a revert of rL257221.
https://reviews.llvm.org/D35879 marks the tests in the lit test suite
that fail on Windows as XFAIL, which should allow these tests to pass
on Windows-based buildbots.

Reviewers: delcypher, beanz, mgorny, jroelofs, rnk

Reviewed By: mgorny

Subscribers: rnk, ddunbar, george.karpenkov, llvm-commits

Differential Revision: https://reviews.llvm.org/D35880

llvm-svn: 309310