Rollup merge of #119841 - nnethercote:rm-DiagnosticBuilder-buffer, r=oli-obk

Remove `DiagnosticBuilder::buffer`

`DiagnosticBuilder::buffer` doesn't do much, and part of what it does (for `-Ztreat-err-as-bug`) it shouldn't.

This PR strips it back, replaces its uses, and finally removes it, making a few cleanups in the vicinity along the way.

r? ``@oli-obk``

Author: matthiaskrgr

clippy_lints/src/doc/needless_doctest_main.rs

@@ -53,7 +53,7 @@ pub fn check(
                 let mut parser = match maybe_new_parser_from_source_str(&sess, filename, code) {
                     Ok(p) => p,
                     Err(errs) => {
-                        drop(errs);
+                        errs.into_iter().for_each(|err| err.cancel());
                         return (false, test_attr_spans);
                     },
                 };