[CMake] Prune LLVM_TOOLS_DIR and LLVM_LIBS_DIR in clang/test/CMakeLists.txt. They are overridden in configure_lit_site_cfg().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199591 91177308-0d34-0410-b5e6-96231b3b80d8