CMake: Include Clang unit tests in check-clang target in standalone builds.

Also, remove CLANG_BUILD_TESTS option. It won't have consistent behavior
between standalone and non-standalone builds, so I'm not going to bother
hooking it up for standalone builds. LLVM_BUILD_TESTS will continue to
control unit test inclusion in the "all" target in non-standalone builds.

Finally, fix the default value of CLANG_INCLUDE_TESTS, which was being set
to the boolean value of "LLVM_INCLUDE_TESTS", i.e. OFF, rather than actually
reading the variable ${LLVM_INCLUDE_TESTS}! If you picked up my earlier
commit, YOU WILL HAVE TO MANUALLY SET THIS OPTION BACK ON. My apologies!

Part two of r174691 (allow the unit tests to be built in standalone mode).