Auto merge of #77255 - Aaron1011:feature/collect-attr-tokens, r=petrochenkov

Unconditionally capture tokens for attributes.

This allows us to avoid synthesizing tokens in `prepend_attr`, since we
have the original tokens available.

We still need to synthesize tokens when expanding `cfg_attr`,
but this is an unavoidable consequence of the syntax of `cfg_attr` -
the user does not supply the `#` and `[]` tokens that a `cfg_attr`
expands to.

This is based on PR https://github.com/rust-lang/rust/pull/77250 - this PR exposes a bug in the current `collect_tokens` implementation, which is fixed by the rewrite.

compiler/rustc_parse/src/parser/expr.rs

@@ -1116,7 +1116,7 @@ impl<'a> Parser<'a> {
     ) -> PResult<'a, P<Expr>> {
         if needs_tokens {
             let (mut expr, tokens) = self.collect_tokens(f)?;
-            expr.tokens = Some(tokens);
+            expr.tokens = tokens;
             Ok(expr)
         } else {
             f(self)