CMake: Optionally allow running the Clang unit tests in standalone builds.

The reason this is not enabled by default is because there is no way for
Clang to guarantee that the LLVM unit testing infrastruture has been built.
However, if it /has/ been built, there's no reason why the standalone Clang
build can't use it!

This should have no effect on existing builds -- in a combined build the
value of the CLANG_INCLUDE_TESTS option defaults to the LLVM equivalent,
and in a standalone build it defaults to off.