[clang-doc][cmake] Copy assets to build directory (llvm#95187)

While we copy the asset files, like index.js, into the correct location
in the install step, tests do not have access to those resources in the
build directory.

This patch copies the contents of the clang-doc/assets directory into
the build folder, so that they can be used in testing.

Pull Request: llvm#95185

Author: ilovepi

clang-tools-extra/clang-doc/tool/CMakeLists.txt

@@ -18,10 +18,38 @@ target_link_libraries(clang-doc
   clangDoc
   )
 
-install(FILES ../assets/clang-doc-default-stylesheet.css
-  DESTINATION "${CMAKE_INSTALL_DATADIR}/clang"
-  COMPONENT clang-doc)
 
-install(FILES ../assets/index.js
-  DESTINATION "${CMAKE_INSTALL_DATADIR}/clang"
-  COMPONENT clang-doc)
+set(assets
+  index.js
+  clang-doc-default-stylesheet.css
+)
+
+set(asset_dir "${CMAKE_CURRENT_SOURCE_DIR}/../assets")
+set(resource_dir "${CMAKE_BINARY_DIR}/share/clang")
+set(out_files)
+
+function(copy_files_to_dst src_dir dst_dir file)
+  set(src "${src_dir}/${file}")
+  set(dst "${dst_dir}/${file}")
+  add_custom_command(OUTPUT ${dst}
+    DEPENDS ${src}
+    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
+    COMMENT "Copying ${file} to ${dst_dir}"
+  )
+  list(APPEND out_files ${dst})
+  set(out_files ${out_files} PARENT_SCOPE)
+endfunction(copy_files_to_dst)
+
+foreach(f ${assets})
+  install(FILES ${asset_dir}/${f}
+    DESTINATION "${CMAKE_INSTALL_DATADIR}/clang"
+    COMPONENT clang-doc)
+  copy_files_to_dst(${asset_dir} ${resource_dir} ${f})
+endforeach(f)
+
+add_custom_target(copy-clang-doc-assets
+  DEPENDS ${out_files}
+  COMMENT "Copying Clang-Doc Assets"
+)
+set_target_properties(copy-clang-doc-assets PROPERTIES FOLDER "Clang-Doc/Assets")
+add_dependencies(clang-doc copy-clang-doc-assets)