Use token::Lit in ast::ExprKind::Lit.

Instead of `ast::Lit`.

Literal lowering now happens at two different times. Expression literals
are lowered when HIR is crated. Attribute literals are lowered during
parsing.

This commit changes the language very slightly. Some programs that used
to not compile now will compile. This is because some invalid literals
that are removed by `cfg` or attribute macros will no longer trigger
errors. See this comment for more details:
https://github.com/rust-lang/rust/pull/102944#issuecomment-1277476773

compiler/rustc_parse/src/lexer/mod.rs

@@ -661,6 +661,7 @@ impl<'a> StringReader<'a> {
         prefix_len: u32,
         postfix_len: u32,
     ) -> (token::LitKind, Symbol) {
+        let mut has_fatal_err = false;
         let content_start = start + BytePos(prefix_len);
         let content_end = end - BytePos(postfix_len);
         let lit_content = self.str_from_to(content_start, content_end);
@@ -672,6 +673,9 @@ impl<'a> StringReader<'a> {
                 let lo = content_start + BytePos(start);
                 let hi = lo + BytePos(end - start);
                 let span = self.mk_sp(lo, hi);
+                if err.is_fatal() {
+                    has_fatal_err = true;
+                }
                 emit_unescape_error(
                     &self.sess.span_diagnostic,
                     lit_content,
@@ -683,7 +687,14 @@ impl<'a> StringReader<'a> {
                 );
             }
         });
-        (kind, Symbol::intern(lit_content))
+
+        // We normally exclude the quotes for the symbol, but for errors we
+        // include it because it results in clearer error messages.
+        if !has_fatal_err {
+            (kind, Symbol::intern(lit_content))
+        } else {
+            (token::Err, self.symbol_from_to(start, end))
+        }
     }
 }