Rollup merge of #122406 - compiler-errors:next-solver-asynckind-wf, r…

…=lcnr

Fix WF for `AsyncFnKindHelper` in new trait solver

`to_opt_closure_kind` ICEs when it sees placeholders... so don't do that

no test b/c I'm too lazy to write a no-core test for this, but I could be convinced otherwise

r? lcnr