Help CMake locate the "tools-extra" directory.

Taken from <https://bugs.llvm.org/show_bug.cgi?id=49990#c3>.

diff --git a/clang-tools-extra/clangd/quality/CompletionModel.cmake b/clang-tools-extra/clangd/quality/CompletionModel.cmake
--- a/clangd/quality/CompletionModel.cmake
+++ b/clangd/quality/CompletionModel.cmake
@@ -5,7 +5,7 @@
 # will define a C++ class called ${cpp_class} - which may be a
 # namespace-qualified class name.
 function(gen_decision_forest model filename cpp_class)
-  set(model_compiler ${CMAKE_SOURCE_DIR}/../clang-tools-extra/clangd/quality/CompletionModelCodegen.py)
+  set(model_compiler ${CMAKE_CURRENT_LIST_DIR}/quality/CompletionModelCodegen.py)
   
   set(output_dir ${CMAKE_CURRENT_BINARY_DIR})
   set(header_file ${output_dir}/${filename}.h)