clang --autocomplete=-std will show
-std: Language standard to compile for -std= Language standard to compile for -stdlib= C++ standard library to use
by this change.
However, showing HelpText with completion in bash seems super tricky, so
this feature will be used in other shells(fish, zsh...).
Now that the separator and the terminator are the same, there's no reason to use llvm::join. Since llvm::join creates a large temporary string, you want to avoid that if it is easily avoidable.
If you do this, you might be able to remove StringExtras.h from this file?