Auto merge of #108138 - compiler-errors:malformed-fn-trait, r=TaKO8Ki

Move `Fn*` traits malformedness protections to typeck

I found it strange that we were doing a custom well-formedness check just for the `Fn*` traits' `call_*` fn items. My understanding from the git history is that this is just to avoid ICEs later on in typeck.

Well, that well-formedness check isn't even implemented correctly for `FnOnce::call_once`, or `FnMut::call_mut` for that matter. Instead, this PR just makes the typeck checks more robust, and leaves it up to the call-site to report errors when lang items are implemented in funny ways.

This coincidentally fixes another ICE where a the `Add` lang item is implemented with a `add` item that's a const instead of a method.

Author: bors