parser: reduce visibility of unnecessary public UnmatchedDelim

`lexer::UnmatchedDelim` struct in `rustc_parse` is unnecessary public
outside of the crate. This commit reduces the visibility to
`pub(crate)`.

Beside, this removes unnecessary field `expected_delim` that causes
warnings after changing the visibility.

Author: ohno418

compiler/rustc_parse/src/lexer/mod.rs

@@ -34,8 +34,7 @@ use unescape_error_reporting::{emit_unescape_error, escaped_char};
 rustc_data_structures::static_assert_size!(rustc_lexer::Token, 12);
 
 #[derive(Clone, Debug)]
-pub struct UnmatchedDelim {
-    pub expected_delim: Delimiter,
+pub(crate) struct UnmatchedDelim {
     pub found_delim: Option<Delimiter>,
     pub found_span: Span,
     pub unclosed_span: Option<Span>,

compiler/rustc_parse/src/lexer/tokentrees.rs

@@ -77,7 +77,6 @@ impl<'psess, 'src> TokenTreesReader<'psess, 'src> {
         for &(_, sp) in &self.diag_info.open_braces {
             err.span_label(sp, "unclosed delimiter");
             self.diag_info.unmatched_delims.push(UnmatchedDelim {
-                expected_delim: Delimiter::Brace,
                 found_delim: None,
                 found_span: self.token.span,
                 unclosed_span: Some(sp),
@@ -163,9 +162,8 @@ impl<'psess, 'src> TokenTreesReader<'psess, 'src> {
                             candidate = Some(*brace_span);
                         }
                     }
-                    let (tok, _) = self.diag_info.open_braces.pop().unwrap();
+                    let (_, _) = self.diag_info.open_braces.pop().unwrap();
                     self.diag_info.unmatched_delims.push(UnmatchedDelim {
-                        expected_delim: tok,
                         found_delim: Some(close_delim),
                         found_span: self.token.span,
                         unclosed_span: unclosed_delimiter,