rustdoc: remove unchecked_claim_error_was_emitted call in main_args.

`main_args` calls `from_matches`, which does lots of initialization. If
anything goes wrong, `from_matches` emits an error message and returns
`Err(1)` (or `Err(3)`). `main_args` then turns the `Err(1)` into
`Err(ErrorGuaranteed)`, because that's what `catch_with_exit_code`
requires on error. But `catch_with_exit_code` doesn't do anything with
the `ErrorGuaranteed`, it just exits with `EXIT_FAILURE`.

We can avoid the creation of the `ErrorGuaranteed` (which requires
an undesirable `unchecked_claim_error_was_emitted` call), by changing
`from_matches` to instead eagerly abort if anything goes wrong. The
behaviour from the user's point of view is the same: an early abort with
an `EXIT_FAILURE` exit code.

And we can also simplify `from_matches` to return an `Option` instead of
a `Result`:
- Old `Err(0)` case --> `None`
- Old `Err(_)` case --> fatal error.

This requires similar changes to `ScrapeExamplesOptions::new` and
`load_call_locations`.

src/librustdoc/lib.rs

@@ -720,15 +720,8 @@ fn main_args(
     // Note that we discard any distinction between different non-zero exit
     // codes from `from_matches` here.
     let (options, render_options) = match config::Options::from_matches(early_dcx, &matches, args) {
-        Ok(opts) => opts,
-        Err(code) => {
-            return if code == 0 {
-                Ok(())
-            } else {
-                #[allow(deprecated)]
-                Err(ErrorGuaranteed::unchecked_claim_error_was_emitted())
-            };
-        }
+        Some(opts) => opts,
+        None => return Ok(()),
     };
 
     let diag =