rustdoc: Support argument files

Factors out the `rustc_driver` logic that handles argument files
so that rustdoc supports them as well, e.g.:

    rustdoc @argfile

This is needed to be able to generate docs for projects that
already use argument files when compiling them, e.g. projects
that pass a huge number of `--cfg` arguments.

The feature was stabilized for `rustc` in #66172.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

src/librustdoc/lib.rs

@@ -432,13 +432,16 @@ fn usage(argv0: &str) {
         (option.apply)(&mut options);
     }
     println!("{}", options.usage(&format!("{} [options] <input>", argv0)));
+    println!("    @path               Read newline separated options from `path`\n");
     println!("More information available at https://doc.rust-lang.org/rustdoc/what-is-rustdoc.html")
 }
 
 /// A result type used by several functions under `main()`.
 type MainResult = Result<(), ErrorReported>;
 
-fn main_args(args: &[String]) -> MainResult {
+fn main_args(at_args: &[String]) -> MainResult {
+    let args = rustc_driver::args::arg_expand_all(at_args);
+
     let mut options = getopts::Options::new();
     for option in opts() {
         (option.apply)(&mut options);