debuginfo: Add a "nodebug" attribute that allows to exclude functions from debuginfo generation.

Right now debuginfo can only be turned on or off globally. Sometimes it would be good to be able to exclude only specific functions from debuginfo, especially when they trigger a bug in rustc's or LLVM's debuginfo handling (e.g. #14930). This way, one would be able to still mostly use debuginfo in the presence of (localized) debuginfo issues. 
