[CMake] AddLLVM.cmake: Tweak the corner case that "check-all" doesn't have any tests.

"check-all" can be executed with 0 status, "check-all does nothing, no tools built."

LLVM_EXTERNAL_CLANG_BUILD=OFF LLVM_BUILD_TOOLS=OFF can reproduce this.
Oscar Fuentes reported this. Thank you.

llvm-svn: 171046