de-orphan extended information

Bizarrely, librustc_passes, librustc_plugin, librustc_mir, and libsyntax
weren't getting their error explanations registered.

Resolves #35284.

src/libsyntax/lib.rs

@@ -148,4 +148,4 @@ pub mod ext {
 #[cfg(test)]
 mod test_snippet;
 
-// __build_diagnostic_array! { libsyntax, DIAGNOSTICS }
+__build_diagnostic_array! { libsyntax, DIAGNOSTICS }