Properly calculate best failure in macro matching

Previously, we used spans. This was not good. Sometimes, the span of the
token that failed to match may come from a position later in the file
which has been transcribed into a token stream way earlier in the file.
If precisely this token fails to match, we think that it was the best
match because its span is so high, even though other arms might have
gotten further in the token stream.

We now try to properly use the location in the token stream.

compiler/rustc_parse/src/parser/mod.rs

@@ -1503,6 +1503,10 @@ impl<'a> Parser<'a> {
     pub fn clear_expected_tokens(&mut self) {
         self.expected_tokens.clear();
     }
+
+    pub fn approx_token_stream_pos(&self) -> usize {
+        self.token_cursor.num_next_calls
+    }
 }
 
 pub(crate) fn make_unclosed_delims_error(