[cmake] Use llvm-lit directory when provided for stand-alone build

After the recent lit test changes, clang attempts to run its tests
via llvm-lit by default. However, the llvm-lit binary is not present
when performing stand-alone build resulting in a failure out of the box.

To solve that, add the llvm-lit directory to CMake when performing
a stand-alone build and LLVM sources are provided. This includes
the CMake rules generating the llvm-lit binary and effectively makes
it possible for clang to use it.

Differential Revision: https://reviews.llvm.org/D40142

llvm-svn: 318562