Auto merge of #116001 - fmease:validate-crate-name-extern-cli-opt, r=est31

[breaking change] Validate crate name in `--extern` [MCP 650]

Reject non-ASCII-identifier crate names passed to the CLI option `--extern` (`rustc`, `rustdoc`).
Implements [MCP 650](https://github.com/rust-lang/compiler-team/issues/650) (except that we only allow ASCII identifiers not arbitrary Rust identifiers).
Fixes #113035.

[As mentioned on Zulip](376826988), doing a crater run probably doesn't make sense since it wouldn't yield anything. Most users don't interact with `rustc` directly but only ever through Cargo which always passes a valid crate name to `--extern` when it invokes `rustc` and `rustdoc`. In any case, the user wouldn't be able to use such a crate name in the source code anyway.

Note that I'm not using [`rustc_session::output::validate_crate_name`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_session/output/fn.validate_crate_name.html) (used for `--crate-name` and `#![crate_name]`) since the latter doesn't reject non-ASCII crate names and ones that start with a digit.

As an aside, I've also thought about getting rid of `validate_crate_name` entirely in a separate PR (with another MCP) in favor of `is_ascii_ident` to reject more weird `--crate-name`s, `#![crate_name]`s and file names but I think that would lead to a lot of actual breakage, namely because of file names starting with a digit. In `tests/ui` 9 tests would be impacted for example.

CC `@estebank`
r? `@est31`

compiler/rustc_session/src/config.rs

@@ -2475,6 +2475,19 @@ pub fn parse_externs(
             Some((opts, name)) => (Some(opts), name.to_string()),
         };
 
+        if !crate::utils::is_ascii_ident(&name) {
+            let mut error = handler.early_struct_error(format!(
+                "crate name `{name}` passed to `--extern` is not a valid ASCII identifier"
+            ));
+            let adjusted_name = name.replace("-", "_");
+            if crate::utils::is_ascii_ident(&adjusted_name) {
+                error.help(format!(
+                    "consider replacing the dashes with underscores: `{adjusted_name}`"
+                ));
+            }
+            error.emit();
+        }
+
         let path = path.map(|p| CanonicalizedPath::new(p));
 
         let entry = externs.entry(name.to_owned());