Stop using DiagnosticBuilder::buffer in the parser.

One consequence is that errors returned by
`maybe_new_parser_from_source_str` now must be consumed, so a bunch of
places that previously ignored those errors now cancel them. (Most of
them explicitly dropped the errors before. I guess that was to indicate
"we are explicitly ignoring these", though I'm not 100% sure.)

Author: nnethercote

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);
                     },
                 };