Tweak expand_incomplete_parse warning.

By using `token_descr`, as is done for many other errors, we can get
slightly better descriptions in error messages, e.g.
"macro expansion ignores token `let` and any following" becomes
"macro expansion ignores keyword `let` and any tokens following".

This will be more important once invisible delimiters start being
mentioned in error messages -- without this commit, that leads to error
messages such as "error at ``" because invisible delimiters are
pretty printed as an empty string.

compiler/rustc_parse/src/parser/mod.rs

@@ -424,7 +424,7 @@ impl TokenDescription {
     }
 }
 
-pub(super) fn token_descr(token: &Token) -> String {
+pub fn token_descr(token: &Token) -> String {
     let name = pprust::token_to_string(token).to_string();
 
     let kind = match (TokenDescription::from_token(token), &token.kind) {