parser: Ensure that all nonterminals have tokens after parsing

[petrochenkov]

parse_nonterminal should always result in something with tokens.

This requirement wasn't satisfied in two cases:

• stmt nonterminal with expression statements (e.g. 0, or {}, or path + 1) because fn parse_stmt_without_recovery forgot to propagate force_collect in some cases.
• expr nonterminal with expressions with built-in attributes (e.g. #[allow(warnings)] 0) due to an incorrect optimization in fn parse_expr_force_collect, it assumed that all expressions starting with # have their tokens collected during parsing, but that's not true if all the attributes on that expression are built-in and inert.

(Discovered when trying to implement eager cfg expansion for all attributes #83824 (comment).)

r? @Aaron1011