Count "unused extern" errors as lints rather than normal errors.

compiler/rustc_errors/src/lib.rs

@@ -1162,7 +1162,7 @@ pub fn emit_unused_externs(
         let mut inner = self.inner.borrow_mut();
 
         if loud && lint_level.is_error() {
-            inner.err_count += 1;
+            inner.lint_err_count += 1;
             inner.panic_if_treat_err_as_bug();
         }