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

Teach the CMake build system to run lit's test suite. These can be run
directy with ``make check-lit`` and are run as part of
``make check-all``.

In principle we should run lit's testsuite before testing LLVM using lit
so that any problems with lit get discovered before testing LLVM so we
can bail out early.  However this implementation (``check-all`` runs all
tests together) seemed simpler and will still report failing lit tests.

Note that the tests and the configured ``lit.site.cfg`` have to be
copied into the build directory to avoid polluting the source tree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257221 91177308-0d34-0410-b5e6-96231b3b80d8