[CMake] Fix an issue building out-of-tree introduced in r272200

The out-of-tree build needs to read LLVM_TOOLS_INSTALL_DIR out of TOOLS_BINARY_DIR because LLVM_TOOLS_INSTALL_DIR is used by AddLLVM.cmake

llvm-svn: 272275

clang/CMakeLists.txt

@@ -56,6 +56,8 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   list(GET CONFIG_OUTPUT 4 LLVM_OBJ_ROOT)
   list(GET CONFIG_OUTPUT 5 MAIN_SRC_DIR)
 
+  get_filename_component(LLVM_TOOLS_INSTALL_DIR ${TOOLS_BINARY_DIR} NAME)
+
   if(NOT MSVC_IDE)
     set(LLVM_ENABLE_ASSERTIONS ${ENABLE_ASSERTIONS}
       CACHE BOOL "Enable assertions")