Potential feature request: integrate Clang's --autocomplete

Apparently, since 2017, Clang has built-in autocomplete support to aid shells: https://blog.llvm.org/2017/09/clang-bash-better-auto-completion-is.html

```bash
> clang --autocomplete=-s
-save-stats Save llvm statistics.
-save-stats= Save llvm statistics.
-save-temps Save intermediate compilation results
-save-temps= Save intermediate compilation results.
-sdl
-sdl-
-serialize-diagnostics Serialize compiler diagnostics to a file
-shared
-shared-libsan Dynamically link the sanitizer runtime
...
```

This currently doesn't work in Emscripten, but could be a helpful addition, especially if it not only forwarded flags but also showed titles for & autocompleted Emscripten's own `-s FOOBAR` settings.