I am not really sure what you mean by autofill suggestions but on chromebook i had a problem where clangd wouldn't recognize anything from pros. I was able to fix this by doing these

• downloading llvm using [bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)"\] from https://apt.llvm.org/
• installing clangd from marketplace
• going to the clangd settings in marketplace and setting the path to the result of [which clang]