[xt] XXX: delay unescaping

Note that from_token_lit was looking for errors but never finding them!

- issue-62913.rs: The structure and output changed a bit. Issue #62913
  was about an ICE due to an unterminated string literal, so the new
  version should be good enough.

- literals-are-validated-before-expansion.rs: this tests exactly the
  behaviour that has been changed.
  XXX: insert a new test covering more of that

- XXX: explain the tests that needed to be split

- XXX: tests/ui/parser/unicode-control-codepoints.rs: just reordered errors

- XXX: tests/rustdoc-ui/ignore-block-help.rs: relies on a parsing error
  occurring. The error present was an unescaping error, which is now
  delayed to after parsing. So the commit changes it to an "unterminated
  character literal" error which still occurs during parsing.

Author: nnethercote

compiler/rustc_ast/src/attr/mod.rs

@@ -240,7 +240,7 @@ impl AttrArgsEq {
         match self {
             AttrArgsEq::Ast(expr) => match expr.kind {
                 ExprKind::Lit(token_lit) => {
-                    LitKind::from_token_lit(token_lit).ok().and_then(|lit| lit.str())
+                    LitKind::from_token_lit(token_lit).0.ok().and_then(|lit| lit.str())
                 }
                 _ => None,
             },
@@ -426,6 +426,7 @@ impl MetaItemKind {
                 ExprKind::Lit(token_lit) => {
                     // Turn failures to `None`, we'll get parse errors elsewhere.
                     MetaItemLit::from_token_lit(token_lit, expr.span)
+                        .0
                         .ok()
                         .map(|lit| MetaItemKind::NameValue(lit))
                 }