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.)

src/librustdoc/clean/render_macro_matchers.rs

@@ -69,8 +69,8 @@ fn snippet_equal_to_token(tcx: TyCtxt<'_>, matcher: &TokenTree) -> Option<String
     let mut parser =
         match rustc_parse::maybe_new_parser_from_source_str(&sess, file_name, snippet.clone()) {
             Ok(parser) => parser,
-            Err(diagnostics) => {
-                drop(diagnostics);
+            Err(errs) => {
+                errs.into_iter().for_each(|err| err.cancel());
                 return None;
             }
         };