Make clangd CompletionModel usable even with non-standard (but supported) layout

[serge-sans-paille]

Summary

llvm supports specifying a non-standard layout where each project lies in its own place. Do not assume a fixed layout and use the appropriate cmake variable instead.

[serge-sans-paille]

@usaxena95 ping?

[usaxena95]

LG. Thanks.
Sorry. This completely missed my inbox.

[wlei]

This comment was removed by wlei.

[glandium]

LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR was removed in https://reviews.llvm.org/D58157, so it's unset by default.

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.
 function(gen_decision_forest model filename cpp_class)
-  set(model_compiler ${CMAKE_SOURCE_DIR}/../clang-tools-extra/clangd/quality/CompletionModelCodegen.py)
-  
+  set(model_compiler ${LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR}/clangd/quality/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}
       --output_dir ${output_dir}