Improve error messages around invalid literals in attribute arguments

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

compiler/rustc_parse/src/parser/expr.rs

@@ -2077,7 +2077,7 @@ impl<'a> Parser<'a> {
         (token::Lit { symbol: name, suffix: None, kind: token::Char }, span)
     }
 
-    pub fn mk_meta_item_lit_char(name: Symbol, span: Span) -> MetaItemLit {
+    fn mk_meta_item_lit_char(name: Symbol, span: Span) -> MetaItemLit {
         ast::MetaItemLit {
             symbol: name,
             suffix: None,
@@ -2086,7 +2086,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    pub fn handle_missing_lit<L>(
+    fn handle_missing_lit<L>(
         &mut self,
         mk_lit_char: impl FnOnce(Symbol, Span) -> L,
     ) -> PResult<'a, L> {