[CMake] fix missing CompletionModelCodegen.py when build clang standalone

[paperchalice]

Summary

When build clang standalone with clang tools extra, cmake reports

ninja: error: '/tmp/clang-20210415-5565-1hte4ld/clang-tools-extra/clangd/quality/CompletionModelCodegen.py', needed by 'tools/extra/clangd/CompletionModel.cpp', missing and no known rule to make it

clang-tools-extra/clangd/quality/CompletionModel.cmake

 # Run the Completion Model Codegenerator on the model present in the 
 # ${model} directory.
 # Produces a pair of files called ${filename}.h and  ${filename}.cpp in the 
 # ${CMAKE_CURRENT_BINARY_DIR}. The generated header
 # will define a C++ class called ${cpp_class} - which may be a
 # namespace-qualified class name.
+
+# TODO: use CMAKE_CURRENT_FUNCTION_LIST_DIR after CMake 3.17
+set(CompletionModelCodegen_py_dir ${CMAKE_CURRENT_LIST_DIR})
 function(gen_decision_forest model filename cpp_class)
-  set(model_compiler ${CMAKE_SOURCE_DIR}/../clang-tools-extra/clangd/quality/CompletionModelCodegen.py)
+  set(model_compiler ${CompletionModelCodegen_py_dir}/CompletionModelCodegen.py)
   
   set(output_dir ${CMAKE_CURRENT_BINARY_DIR})
   set(header_file ${output_dir}/${filename}.h)
   set(cpp_file ${output_dir}/${filename}.cpp)
 
   add_custom_command(OUTPUT ${header_file} ${cpp_file}
     COMMAND "${Python3_EXECUTABLE}" ${model_compiler}
       --model ${model}